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<button aria-controls="toc-Types_and_zygosity-sublist" class="cdx-button cdx-button--weight-quiet cdx-button--icon-only vector-toc-toggle"> <span class="vector-icon mw-ui-icon-wikimedia-expand"></span> <span>Toggle Types and zygosity subsection</span> </button> <ul id="toc-Types_and_zygosity-sublist" class="vector-toc-list"> <li id="toc-Dizygotic_(fraternal)_twins" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Dizygotic_(fraternal)_twins"> <div class="vector-toc-text"> <span class="vector-toc-numb">2.1</span> <span>Dizygotic (fraternal) twins</span> </div> </a> <ul id="toc-Dizygotic_(fraternal)_twins-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Monozygotic_(identical)_twins" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Monozygotic_(identical)_twins"> <div class="vector-toc-text"> <span class="vector-toc-numb">2.2</span> <span>Monozygotic (identical) twins</span> </div> </a> <ul id="toc-Monozygotic_(identical)_twins-sublist" class="vector-toc-list"> <li id="toc-Mechanism" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#Mechanism"> <div class="vector-toc-text"> <span class="vector-toc-numb">2.2.1</span> <span>Mechanism</span> </div> </a> <ul id="toc-Mechanism-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Incidence" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#Incidence"> <div class="vector-toc-text"> <span class="vector-toc-numb">2.2.2</span> <span>Incidence</span> </div> </a> <ul id="toc-Incidence-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Genetic_and_epigenetic_similarity" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#Genetic_and_epigenetic_similarity"> <div class="vector-toc-text"> <span class="vector-toc-numb">2.2.3</span> <span>Genetic and epigenetic similarity</span> </div> </a> <ul id="toc-Genetic_and_epigenetic_similarity-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-Polar_body_and_semi-identical_twins" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Polar_body_and_semi-identical_twins"> <div class="vector-toc-text"> <span class="vector-toc-numb">2.3</span> <span>Polar body and semi-identical twins</span> </div> </a> <ul id="toc-Polar_body_and_semi-identical_twins-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-Degree_of_separation" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#Degree_of_separation"> <div class="vector-toc-text"> <span class="vector-toc-numb">3</span> <span>Degree of separation</span> </div> </a> <ul id="toc-Degree_of_separation-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Demographics" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#Demographics"> <div class="vector-toc-text"> <span class="vector-toc-numb">4</span> <span>Demographics</span> </div> </a> <button aria-controls="toc-Demographics-sublist" class="cdx-button cdx-button--weight-quiet cdx-button--icon-only vector-toc-toggle"> <span class="vector-icon mw-ui-icon-wikimedia-expand"></span> <span>Toggle Demographics subsection</span> </button> <ul id="toc-Demographics-sublist" class="vector-toc-list"> <li id="toc-Ethnicity" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Ethnicity"> <div class="vector-toc-text"> <span class="vector-toc-numb">4.1</span> <span>Ethnicity</span> </div> </a> <ul id="toc-Ethnicity-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Predisposing_factors" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Predisposing_factors"> <div class="vector-toc-text"> <span class="vector-toc-numb">4.2</span> <span>Predisposing factors</span> </div> </a> <ul id="toc-Predisposing_factors-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-Delivery_interval" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#Delivery_interval"> <div class="vector-toc-text"> <span class="vector-toc-numb">5</span> <span>Delivery interval</span> </div> </a> <ul id="toc-Delivery_interval-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Complications_during_pregnancy" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#Complications_during_pregnancy"> <div class="vector-toc-text"> <span class="vector-toc-numb">6</span> <span>Complications during pregnancy</span> </div> </a> <button aria-controls="toc-Complications_during_pregnancy-sublist" class="cdx-button cdx-button--weight-quiet cdx-button--icon-only vector-toc-toggle"> <span class="vector-icon mw-ui-icon-wikimedia-expand"></span> <span>Toggle Complications during pregnancy subsection</span> </button> <ul id="toc-Complications_during_pregnancy-sublist" class="vector-toc-list"> <li id="toc-Vanishing_twins" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Vanishing_twins"> <div class="vector-toc-text"> <span class="vector-toc-numb">6.1</span> <span>Vanishing twins</span> </div> </a> <ul id="toc-Vanishing_twins-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Conjoined_twins" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Conjoined_twins"> <div class="vector-toc-text"> <span class="vector-toc-numb">6.2</span> <span>Conjoined twins</span> </div> </a> <ul id="toc-Conjoined_twins-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Chimerism" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Chimerism"> <div class="vector-toc-text"> <span class="vector-toc-numb">6.3</span> <span>Chimerism</span> </div> </a> <ul id="toc-Chimerism-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Parasitic_twins" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Parasitic_twins"> <div class="vector-toc-text"> <span class="vector-toc-numb">6.4</span> <span>Parasitic twins</span> </div> </a> <ul id="toc-Parasitic_twins-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Partial_molar_twins" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Partial_molar_twins"> <div class="vector-toc-text"> <span class="vector-toc-numb">6.5</span> <span>Partial molar twins</span> </div> </a> <ul id="toc-Partial_molar_twins-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Miscarried_twin" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Miscarried_twin"> <div class="vector-toc-text"> <span class="vector-toc-numb">6.6</span> <span>Miscarried twin</span> </div> </a> <ul id="toc-Miscarried_twin-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Low_birth_weight" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Low_birth_weight"> <div class="vector-toc-text"> <span class="vector-toc-numb">6.7</span> <span>Low birth weight</span> </div> </a> <ul id="toc-Low_birth_weight-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Twin-to-twin_transfusion_syndrome" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Twin-to-twin_transfusion_syndrome"> <div class="vector-toc-text"> <span class="vector-toc-numb">6.8</span> <span>Twin-to-twin transfusion syndrome</span> </div> </a> <ul id="toc-Twin-to-twin_transfusion_syndrome-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Stillbirths" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Stillbirths"> <div class="vector-toc-text"> <span class="vector-toc-numb">6.9</span> <span>Stillbirths</span> </div> </a> <ul id="toc-Stillbirths-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Heterotopic_pregnancy" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Heterotopic_pregnancy"> <div class="vector-toc-text"> <span class="vector-toc-numb">6.10</span> <span>Heterotopic pregnancy</span> </div> </a> <ul id="toc-Heterotopic_pregnancy-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-Management_of_birth" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#Management_of_birth"> <div class="vector-toc-text"> <span class="vector-toc-numb">7</span> <span>Management of birth</span> </div> </a> <ul id="toc-Management_of_birth-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Human_twin_studies" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#Human_twin_studies"> <div class="vector-toc-text"> <span class="vector-toc-numb">8</span> <span>Human twin studies</span> </div> </a> <ul id="toc-Human_twin_studies-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Unusual_twinnings" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#Unusual_twinnings"> <div class="vector-toc-text"> <span class="vector-toc-numb">9</span> <span>Unusual twinnings</span> </div> </a> <button aria-controls="toc-Unusual_twinnings-sublist" class="cdx-button cdx-button--weight-quiet cdx-button--icon-only vector-toc-toggle"> <span class="vector-icon mw-ui-icon-wikimedia-expand"></span> <span>Toggle Unusual twinnings subsection</span> </button> <ul id="toc-Unusual_twinnings-sublist" class="vector-toc-list"> <li id="toc-Bi-paternal_twins" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Bi-paternal_twins"> <div class="vector-toc-text"> <span class="vector-toc-numb">9.1</span> <span>Bi-paternal twins</span> </div> </a> <ul id="toc-Bi-paternal_twins-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Mixed_twins" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Mixed_twins"> <div class="vector-toc-text"> <span class="vector-toc-numb">9.2</span> <span>Mixed twins</span> </div> </a> <ul id="toc-Mixed_twins-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Monozygotic_twins_of_different_sexes" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Monozygotic_twins_of_different_sexes"> <div class="vector-toc-text"> <span class="vector-toc-numb">9.3</span> <span>Monozygotic twins of different sexes</span> </div> </a> <ul id="toc-Monozygotic_twins_of_different_sexes-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Semi-identical_(sesquizygotic)_twins" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Semi-identical_(sesquizygotic)_twins"> <div class="vector-toc-text"> <span class="vector-toc-numb">9.4</span> <span>Semi-identical (sesquizygotic) twins</span> </div> </a> <ul id="toc-Semi-identical_(sesquizygotic)_twins-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Mirror_image_twins" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Mirror_image_twins"> <div class="vector-toc-text"> <span class="vector-toc-numb">9.5</span> <span>Mirror image twins</span> </div> </a> <ul id="toc-Mirror_image_twins-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-Language_development" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#Language_development"> <div class="vector-toc-text"> <span class="vector-toc-numb">10</span> <span>Language development</span> </div> </a> <ul id="toc-Language_development-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Animals" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#Animals"> <div class="vector-toc-text"> <span class="vector-toc-numb">11</span> <span>Animals</span> </div> </a> <ul id="toc-Animals-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-See_also" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#See_also"> <div class="vector-toc-text"> <span class="vector-toc-numb">12</span> <span>See also</span> </div> </a> <ul id="toc-See_also-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-References" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#References"> <div class="vector-toc-text"> <span class="vector-toc-numb">13</span> <span>References</span> </div> </a> <ul id="toc-References-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Further_reading" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#Further_reading"> <div class="vector-toc-text"> <span class="vector-toc-numb">14</span> <span>Further reading</span> </div> </a> <ul id="toc-Further_reading-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-External_links" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#External_links"> <div class="vector-toc-text"> <span class="vector-toc-numb">15</span> <span>External links</span> </div> </a> <ul id="toc-External_links-sublist" class="vector-toc-list"> </ul> </li> </ul> </div> </div> </nav> </div> </div> <div class="mw-content-container"> <main id="content" class="mw-body"> <header class="mw-body-header vector-page-titlebar"> <nav aria-label="Contents" class="vector-toc-landmark"> <div id="vector-page-titlebar-toc" class="vector-dropdown vector-page-titlebar-toc vector-button-flush-left" > <input type="checkbox" id="vector-page-titlebar-toc-checkbox" role="button" aria-haspopup="true" data-event-name="ui.dropdown-vector-page-titlebar-toc" class="vector-dropdown-checkbox " aria-label="Toggle the table of contents" > <label id="vector-page-titlebar-toc-label" for="vector-page-titlebar-toc-checkbox" class="vector-dropdown-label cdx-button cdx-button--fake-button cdx-button--fake-button--enabled cdx-button--weight-quiet cdx-button--icon-only " aria-hidden="true" ><span class="vector-icon mw-ui-icon-listBullet mw-ui-icon-wikimedia-listBullet"></span> <span class="vector-dropdown-label-text">Toggle the table of contents</span> </label> <div class="vector-dropdown-content"> <div id="vector-page-titlebar-toc-unpinned-container" class="vector-unpinned-container"> </div> </div> </div> </nav> <h1 id="firstHeading" class="firstHeading mw-first-heading"><span class="mw-page-title-main">Twin</span></h1> <div id="p-lang-btn" class="vector-dropdown mw-portlet mw-portlet-lang" > <input type="checkbox" id="p-lang-btn-checkbox" role="button" aria-haspopup="true" data-event-name="ui.dropdown-p-lang-btn" class="vector-dropdown-checkbox mw-interlanguage-selector" aria-label="Go to an article in another language. Available in 97 languages" > <label id="p-lang-btn-label" for="p-lang-btn-checkbox" class="vector-dropdown-label cdx-button cdx-button--fake-button cdx-button--fake-button--enabled cdx-button--weight-quiet cdx-button--action-progressive mw-portlet-lang-heading-97" aria-hidden="true" ><span class="vector-icon mw-ui-icon-language-progressive mw-ui-icon-wikimedia-language-progressive"></span> <span class="vector-dropdown-label-text">97 languages</span> </label> <div class="vector-dropdown-content"> <div class="vector-menu-content"> <ul class="vector-menu-content-list"> <li class="interlanguage-link interwiki-af mw-list-item"><a href="https://af.wikipedia.org/wiki/Tweeling" title="Tweeling – Afrikaans" lang="af" hreflang="af" data-title="Tweeling" data-language-autonym="Afrikaans" data-language-local-name="Afrikaans" class="interlanguage-link-target"><span>Afrikaans</span></a></li><li class="interlanguage-link interwiki-ar mw-list-item"><a href="https://ar.wikipedia.org/wiki/%D8%AA%D9%88%D8%A3%D9%85" title="توأم – Arabic" lang="ar" hreflang="ar" data-title="توأم" data-language-autonym="العربية" data-language-local-name="Arabic" class="interlanguage-link-target"><span>العربية</span></a></li><li class="interlanguage-link interwiki-an mw-list-item"><a href="https://an.wikipedia.org/wiki/Melguizo" title="Melguizo – Aragonese" lang="an" hreflang="an" data-title="Melguizo" data-language-autonym="Aragonés" data-language-local-name="Aragonese" class="interlanguage-link-target"><span>Aragonés</span></a></li><li class="interlanguage-link interwiki-arc mw-list-item"><a href="https://arc.wikipedia.org/wiki/%DC%AC%DC%90%DC%A1%DC%90" title="ܬܐܡܐ – Aramaic" lang="arc" hreflang="arc" data-title="ܬܐܡܐ" data-language-autonym="ܐܪܡܝܐ" data-language-local-name="Aramaic" class="interlanguage-link-target"><span>ܐܪܡܝܐ</span></a></li><li class="interlanguage-link interwiki-ast mw-list-item"><a href="https://ast.wikipedia.org/wiki/Ximielgu" title="Ximielgu – Asturian" lang="ast" hreflang="ast" data-title="Ximielgu" data-language-autonym="Asturianu" data-language-local-name="Asturian" class="interlanguage-link-target"><span>Asturianu</span></a></li><li class="interlanguage-link interwiki-gn mw-list-item"><a href="https://gn.wikipedia.org/wiki/Membyk%C3%B5i" title="Membykõi – Guarani" lang="gn" hreflang="gn" data-title="Membykõi" data-language-autonym="Avañe'ẽ" data-language-local-name="Guarani" class="interlanguage-link-target"><span>Avañe'ẽ</span></a></li><li class="interlanguage-link interwiki-az mw-list-item"><a href="https://az.wikipedia.org/wiki/%C6%8Fkiz" title="Əkiz – Azerbaijani" lang="az" hreflang="az" data-title="Əkiz" data-language-autonym="Azərbaycanca" data-language-local-name="Azerbaijani" class="interlanguage-link-target"><span>Azərbaycanca</span></a></li><li class="interlanguage-link interwiki-bn mw-list-item"><a href="https://bn.wikipedia.org/wiki/%E0%A6%AF%E0%A6%AE%E0%A6%9C" title="যমজ – Bangla" lang="bn" hreflang="bn" data-title="যমজ" data-language-autonym="বাংলা" data-language-local-name="Bangla" class="interlanguage-link-target"><span>বাংলা</span></a></li><li class="interlanguage-link interwiki-be mw-list-item"><a href="https://be.wikipedia.org/wiki/%D0%91%D0%BB%D1%96%D0%B7%D0%BD%D1%8F%D1%82%D1%8B" title="Блізняты – Belarusian" lang="be" hreflang="be" data-title="Блізняты" data-language-autonym="Беларуская" data-language-local-name="Belarusian" class="interlanguage-link-target"><span>Беларуская</span></a></li><li class="interlanguage-link interwiki-bcl mw-list-item"><a href="https://bcl.wikipedia.org/wiki/Kambal" title="Kambal – Central Bikol" lang="bcl" hreflang="bcl" data-title="Kambal" data-language-autonym="Bikol Central" data-language-local-name="Central Bikol" class="interlanguage-link-target"><span>Bikol Central</span></a></li><li class="interlanguage-link interwiki-bg mw-list-item"><a href="https://bg.wikipedia.org/wiki/%D0%91%D0%BB%D0%B8%D0%B7%D0%BD%D0%B0%D1%86%D0%B8" title="Близнаци – Bulgarian" lang="bg" hreflang="bg" data-title="Близнаци" data-language-autonym="Български" data-language-local-name="Bulgarian" class="interlanguage-link-target"><span>Български</span></a></li><li class="interlanguage-link interwiki-bar mw-list-item"><a href="https://bar.wikipedia.org/wiki/Zwinling" title="Zwinling – Bavarian" lang="bar" hreflang="bar" data-title="Zwinling" data-language-autonym="Boarisch" data-language-local-name="Bavarian" class="interlanguage-link-target"><span>Boarisch</span></a></li><li class="interlanguage-link interwiki-bs mw-list-item"><a href="https://bs.wikipedia.org/wiki/Blizanac" title="Blizanac – Bosnian" lang="bs" hreflang="bs" data-title="Blizanac" data-language-autonym="Bosanski" data-language-local-name="Bosnian" class="interlanguage-link-target"><span>Bosanski</span></a></li><li class="interlanguage-link interwiki-br mw-list-item"><a href="https://br.wikipedia.org/wiki/Gevell" title="Gevell – Breton" lang="br" hreflang="br" data-title="Gevell" data-language-autonym="Brezhoneg" data-language-local-name="Breton" class="interlanguage-link-target"><span>Brezhoneg</span></a></li><li class="interlanguage-link interwiki-ca mw-list-item"><a href="https://ca.wikipedia.org/wiki/Bess%C3%B3" title="Bessó – Catalan" lang="ca" hreflang="ca" data-title="Bessó" data-language-autonym="Català" data-language-local-name="Catalan" class="interlanguage-link-target"><span>Català</span></a></li><li class="interlanguage-link interwiki-cs mw-list-item"><a href="https://cs.wikipedia.org/wiki/Dvoj%C4%8Data" title="Dvojčata – Czech" lang="cs" hreflang="cs" data-title="Dvojčata" data-language-autonym="Čeština" data-language-local-name="Czech" class="interlanguage-link-target"><span>Čeština</span></a></li><li class="interlanguage-link interwiki-sn mw-list-item"><a href="https://sn.wikipedia.org/wiki/Mapatya" title="Mapatya – Shona" lang="sn" hreflang="sn" data-title="Mapatya" data-language-autonym="ChiShona" data-language-local-name="Shona" class="interlanguage-link-target"><span>ChiShona</span></a></li><li class="interlanguage-link interwiki-cy mw-list-item"><a href="https://cy.wikipedia.org/wiki/Gefell" title="Gefell – Welsh" lang="cy" hreflang="cy" data-title="Gefell" data-language-autonym="Cymraeg" data-language-local-name="Welsh" class="interlanguage-link-target"><span>Cymraeg</span></a></li><li class="interlanguage-link interwiki-da mw-list-item"><a href="https://da.wikipedia.org/wiki/Tvilling" title="Tvilling – Danish" lang="da" hreflang="da" data-title="Tvilling" data-language-autonym="Dansk" data-language-local-name="Danish" class="interlanguage-link-target"><span>Dansk</span></a></li><li class="interlanguage-link interwiki-de mw-list-item"><a href="https://de.wikipedia.org/wiki/Zwillinge" title="Zwillinge – German" lang="de" hreflang="de" data-title="Zwillinge" data-language-autonym="Deutsch" data-language-local-name="German" class="interlanguage-link-target"><span>Deutsch</span></a></li><li class="interlanguage-link interwiki-et mw-list-item"><a href="https://et.wikipedia.org/wiki/Kaksikud" title="Kaksikud – Estonian" lang="et" hreflang="et" data-title="Kaksikud" data-language-autonym="Eesti" data-language-local-name="Estonian" class="interlanguage-link-target"><span>Eesti</span></a></li><li class="interlanguage-link interwiki-el mw-list-item"><a href="https://el.wikipedia.org/wiki/%CE%94%CE%AF%CE%B4%CF%85%CE%BC%CE%BF%CF%82" title="Δίδυμος – Greek" lang="el" hreflang="el" data-title="Δίδυμος" data-language-autonym="Ελληνικά" data-language-local-name="Greek" class="interlanguage-link-target"><span>Ελληνικά</span></a></li><li class="interlanguage-link interwiki-es mw-list-item"><a href="https://es.wikipedia.org/wiki/Gemelo_(biolog%C3%ADa)" title="Gemelo (biología) – Spanish" lang="es" hreflang="es" data-title="Gemelo (biología)" data-language-autonym="Español" data-language-local-name="Spanish" class="interlanguage-link-target"><span>Español</span></a></li><li class="interlanguage-link interwiki-eo mw-list-item"><a href="https://eo.wikipedia.org/wiki/%C4%9Cemelo" title="Ĝemelo – Esperanto" lang="eo" hreflang="eo" data-title="Ĝemelo" data-language-autonym="Esperanto" data-language-local-name="Esperanto" class="interlanguage-link-target"><span>Esperanto</span></a></li><li class="interlanguage-link interwiki-eu mw-list-item"><a href="https://eu.wikipedia.org/wiki/Biki" title="Biki – Basque" lang="eu" hreflang="eu" data-title="Biki" data-language-autonym="Euskara" data-language-local-name="Basque" class="interlanguage-link-target"><span>Euskara</span></a></li><li class="interlanguage-link interwiki-fa mw-list-item"><a href="https://fa.wikipedia.org/wiki/%D8%AF%D9%88%D9%82%D9%84%D9%88" title="دوقلو – Persian" lang="fa" hreflang="fa" data-title="دوقلو" data-language-autonym="فارسی" data-language-local-name="Persian" class="interlanguage-link-target"><span>فارسی</span></a></li><li class="interlanguage-link interwiki-fr mw-list-item"><a href="https://fr.wikipedia.org/wiki/Jumeau" title="Jumeau – French" lang="fr" hreflang="fr" data-title="Jumeau" data-language-autonym="Français" data-language-local-name="French" class="interlanguage-link-target"><span>Français</span></a></li><li class="interlanguage-link interwiki-ga mw-list-item"><a href="https://ga.wikipedia.org/wiki/C%C3%BApla_(daoine)" title="Cúpla (daoine) – Irish" lang="ga" hreflang="ga" data-title="Cúpla (daoine)" data-language-autonym="Gaeilge" data-language-local-name="Irish" class="interlanguage-link-target"><span>Gaeilge</span></a></li><li class="interlanguage-link interwiki-gd mw-list-item"><a href="https://gd.wikipedia.org/wiki/Leth-aon" title="Leth-aon – Scottish Gaelic" lang="gd" hreflang="gd" data-title="Leth-aon" data-language-autonym="Gàidhlig" data-language-local-name="Scottish Gaelic" class="interlanguage-link-target"><span>Gàidhlig</span></a></li><li class="interlanguage-link interwiki-gl mw-list-item"><a href="https://gl.wikipedia.org/wiki/Xemelgo" title="Xemelgo – Galician" lang="gl" hreflang="gl" data-title="Xemelgo" data-language-autonym="Galego" data-language-local-name="Galician" class="interlanguage-link-target"><span>Galego</span></a></li><li class="interlanguage-link interwiki-ko mw-list-item"><a href="https://ko.wikipedia.org/wiki/%EC%8C%8D%EB%91%A5%EC%9D%B4" title="쌍둥이 – Korean" lang="ko" hreflang="ko" data-title="쌍둥이" data-language-autonym="한국어" data-language-local-name="Korean" class="interlanguage-link-target"><span>한국어</span></a></li><li class="interlanguage-link interwiki-ha mw-list-item"><a href="https://ha.wikipedia.org/wiki/Tagwaye" title="Tagwaye – Hausa" lang="ha" hreflang="ha" data-title="Tagwaye" data-language-autonym="Hausa" data-language-local-name="Hausa" class="interlanguage-link-target"><span>Hausa</span></a></li><li class="interlanguage-link interwiki-hy mw-list-item"><a href="https://hy.wikipedia.org/wiki/%D4%B5%D6%80%D5%AF%D5%BE%D5%B8%D6%80%D5%B5%D5%A1%D5%AF%D5%B6%D5%A5%D6%80" title="Երկվորյակներ – Armenian" lang="hy" hreflang="hy" data-title="Երկվորյակներ" data-language-autonym="Հայերեն" data-language-local-name="Armenian" class="interlanguage-link-target"><span>Հայերեն</span></a></li><li class="interlanguage-link interwiki-hi mw-list-item"><a href="https://hi.wikipedia.org/wiki/%E0%A4%9C%E0%A5%81%E0%A4%A1%E0%A4%BC%E0%A4%B5%E0%A4%BE%E0%A4%81" title="जुड़वाँ – Hindi" lang="hi" hreflang="hi" data-title="जुड़वाँ" data-language-autonym="हिन्दी" data-language-local-name="Hindi" class="interlanguage-link-target"><span>हिन्दी</span></a></li><li class="interlanguage-link interwiki-hr mw-list-item"><a href="https://hr.wikipedia.org/wiki/Blizanci" title="Blizanci – Croatian" lang="hr" hreflang="hr" data-title="Blizanci" data-language-autonym="Hrvatski" data-language-local-name="Croatian" class="interlanguage-link-target"><span>Hrvatski</span></a></li><li class="interlanguage-link interwiki-io mw-list-item"><a href="https://io.wikipedia.org/wiki/Jemelo" title="Jemelo – Ido" lang="io" hreflang="io" data-title="Jemelo" data-language-autonym="Ido" data-language-local-name="Ido" class="interlanguage-link-target"><span>Ido</span></a></li><li class="interlanguage-link interwiki-id mw-list-item"><a href="https://id.wikipedia.org/wiki/Kembar" title="Kembar – Indonesian" lang="id" hreflang="id" data-title="Kembar" data-language-autonym="Bahasa Indonesia" data-language-local-name="Indonesian" class="interlanguage-link-target"><span>Bahasa Indonesia</span></a></li><li class="interlanguage-link interwiki-is mw-list-item"><a href="https://is.wikipedia.org/wiki/Tv%C3%ADburi" title="Tvíburi – Icelandic" lang="is" hreflang="is" data-title="Tvíburi" data-language-autonym="Íslenska" data-language-local-name="Icelandic" class="interlanguage-link-target"><span>Íslenska</span></a></li><li class="interlanguage-link interwiki-it mw-list-item"><a href="https://it.wikipedia.org/wiki/Gemelli_(biologia)" title="Gemelli (biologia) – Italian" lang="it" hreflang="it" data-title="Gemelli (biologia)" data-language-autonym="Italiano" data-language-local-name="Italian" class="interlanguage-link-target"><span>Italiano</span></a></li><li class="interlanguage-link interwiki-he mw-list-item"><a href="https://he.wikipedia.org/wiki/%D7%AA%D7%90%D7%95%D7%9E%D7%99%D7%9D" title="תאומים – Hebrew" lang="he" hreflang="he" data-title="תאומים" data-language-autonym="עברית" data-language-local-name="Hebrew" class="interlanguage-link-target"><span>עברית</span></a></li><li class="interlanguage-link interwiki-kn mw-list-item"><a href="https://kn.wikipedia.org/wiki/%E0%B2%85%E0%B2%B5%E0%B2%B3%E0%B2%BF_%E0%B2%9C%E0%B2%B5%E0%B2%B3%E0%B2%BF(%E0%B2%9C%E0%B2%A8%E0%B2%BF%E0%B2%B8%E0%B3%81%E0%B2%B5%E0%B2%BF%E0%B2%95%E0%B3%86)" title="ಅವಳಿ ಜವಳಿ(ಜನಿಸುವಿಕೆ) – Kannada" lang="kn" hreflang="kn" data-title="ಅವಳಿ ಜವಳಿ(ಜನಿಸುವಿಕೆ)" data-language-autonym="ಕನ್ನಡ" data-language-local-name="Kannada" class="interlanguage-link-target"><span>ಕನ್ನಡ</span></a></li><li class="interlanguage-link interwiki-ks mw-list-item"><a href="https://ks.wikipedia.org/wiki/%D8%AF%D9%8F%DA%A9%DB%81%D9%90" title="دُکہِ – Kashmiri" lang="ks" hreflang="ks" data-title="دُکہِ" data-language-autonym="कॉशुर / کٲشُر" data-language-local-name="Kashmiri" class="interlanguage-link-target"><span>कॉशुर / کٲشُر</span></a></li><li class="interlanguage-link interwiki-kk mw-list-item"><a href="https://kk.wikipedia.org/wiki/%D0%95%D0%B3%D1%96%D0%B7" title="Егіз – Kazakh" lang="kk" hreflang="kk" data-title="Егіз" data-language-autonym="Қазақша" data-language-local-name="Kazakh" class="interlanguage-link-target"><span>Қазақша</span></a></li><li class="interlanguage-link interwiki-sw mw-list-item"><a href="https://sw.wikipedia.org/wiki/Mapacha" title="Mapacha – Swahili" lang="sw" hreflang="sw" data-title="Mapacha" data-language-autonym="Kiswahili" data-language-local-name="Swahili" class="interlanguage-link-target"><span>Kiswahili</span></a></li><li class="interlanguage-link interwiki-ht mw-list-item"><a href="https://ht.wikipedia.org/wiki/Jimo" title="Jimo – Haitian Creole" lang="ht" hreflang="ht" data-title="Jimo" data-language-autonym="Kreyòl ayisyen" data-language-local-name="Haitian Creole" class="interlanguage-link-target"><span>Kreyòl ayisyen</span></a></li><li class="interlanguage-link interwiki-ky mw-list-item"><a href="https://ky.wikipedia.org/wiki/%D0%AD%D0%B3%D0%B8%D0%B7%D0%B4%D0%B5%D1%80" title="Эгиздер – Kyrgyz" lang="ky" hreflang="ky" data-title="Эгиздер" data-language-autonym="Кыргызча" data-language-local-name="Kyrgyz" class="interlanguage-link-target"><span>Кыргызча</span></a></li><li class="interlanguage-link interwiki-lad mw-list-item"><a href="https://lad.wikipedia.org/wiki/Buchukes_(biolojiya)" title="Buchukes (biolojiya) – Ladino" lang="lad" hreflang="lad" data-title="Buchukes (biolojiya)" data-language-autonym="Ladino" data-language-local-name="Ladino" class="interlanguage-link-target"><span>Ladino</span></a></li><li class="interlanguage-link interwiki-la mw-list-item"><a href="https://la.wikipedia.org/wiki/Gemini" title="Gemini – Latin" lang="la" hreflang="la" data-title="Gemini" data-language-autonym="Latina" data-language-local-name="Latin" class="interlanguage-link-target"><span>Latina</span></a></li><li class="interlanguage-link interwiki-lv mw-list-item"><a href="https://lv.wikipedia.org/wiki/Dv%C4%AB%C5%86i" title="Dvīņi – Latvian" lang="lv" hreflang="lv" data-title="Dvīņi" data-language-autonym="Latviešu" data-language-local-name="Latvian" class="interlanguage-link-target"><span>Latviešu</span></a></li><li class="interlanguage-link interwiki-lb mw-list-item"><a href="https://lb.wikipedia.org/wiki/Zwilling" title="Zwilling – Luxembourgish" lang="lb" hreflang="lb" data-title="Zwilling" data-language-autonym="Lëtzebuergesch" data-language-local-name="Luxembourgish" class="interlanguage-link-target"><span>Lëtzebuergesch</span></a></li><li class="interlanguage-link interwiki-lt mw-list-item"><a href="https://lt.wikipedia.org/wiki/Dvyniai" title="Dvyniai – Lithuanian" lang="lt" hreflang="lt" data-title="Dvyniai" data-language-autonym="Lietuvių" data-language-local-name="Lithuanian" class="interlanguage-link-target"><span>Lietuvių</span></a></li><li class="interlanguage-link interwiki-ln mw-list-item"><a href="https://ln.wikipedia.org/wiki/Lip%C3%A1sa" title="Lipása – Lingala" lang="ln" hreflang="ln" data-title="Lipása" data-language-autonym="Lingála" data-language-local-name="Lingala" class="interlanguage-link-target"><span>Lingála</span></a></li><li class="interlanguage-link interwiki-hu mw-list-item"><a href="https://hu.wikipedia.org/wiki/Ikrek" title="Ikrek – Hungarian" lang="hu" hreflang="hu" data-title="Ikrek" data-language-autonym="Magyar" data-language-local-name="Hungarian" class="interlanguage-link-target"><span>Magyar</span></a></li><li class="interlanguage-link interwiki-mg mw-list-item"><a href="https://mg.wikipedia.org/wiki/Kambana" title="Kambana – Malagasy" lang="mg" hreflang="mg" data-title="Kambana" data-language-autonym="Malagasy" data-language-local-name="Malagasy" class="interlanguage-link-target"><span>Malagasy</span></a></li><li class="interlanguage-link interwiki-ml mw-list-item"><a href="https://ml.wikipedia.org/wiki/%E0%B4%87%E0%B4%B0%E0%B4%9F%E0%B5%8D%E0%B4%9F%E0%B4%95%E0%B5%8D%E0%B4%95%E0%B5%81%E0%B4%9F%E0%B5%8D%E0%B4%9F%E0%B4%BF%E0%B4%95%E0%B5%BE" title="ഇരട്ടക്കുട്ടികൾ – Malayalam" lang="ml" hreflang="ml" data-title="ഇരട്ടക്കുട്ടികൾ" data-language-autonym="മലയാളം" data-language-local-name="Malayalam" class="interlanguage-link-target"><span>മലയാളം</span></a></li><li class="interlanguage-link interwiki-mr mw-list-item"><a href="https://mr.wikipedia.org/wiki/%E0%A4%9C%E0%A5%81%E0%A4%B3%E0%A5%87" title="जुळे – Marathi" lang="mr" hreflang="mr" data-title="जुळे" data-language-autonym="मराठी" data-language-local-name="Marathi" class="interlanguage-link-target"><span>मराठी</span></a></li><li class="interlanguage-link interwiki-ms mw-list-item"><a href="https://ms.wikipedia.org/wiki/Kembar" title="Kembar – Malay" lang="ms" hreflang="ms" data-title="Kembar" data-language-autonym="Bahasa Melayu" data-language-local-name="Malay" class="interlanguage-link-target"><span>Bahasa Melayu</span></a></li><li class="interlanguage-link interwiki-min mw-list-item"><a href="https://min.wikipedia.org/wiki/Kamba" title="Kamba – Minangkabau" lang="min" hreflang="min" data-title="Kamba" data-language-autonym="Minangkabau" data-language-local-name="Minangkabau" class="interlanguage-link-target"><span>Minangkabau</span></a></li><li class="interlanguage-link interwiki-cdo mw-list-item"><a href="https://cdo.wikipedia.org/wiki/S%C4%95%CC%A4ng-s%C4%83ng-gi%C4%81ng" title="Sĕ̤ng-săng-giāng – Mindong" lang="cdo" hreflang="cdo" data-title="Sĕ̤ng-săng-giāng" data-language-autonym="閩東語 / Mìng-dĕ̤ng-ngṳ̄" data-language-local-name="Mindong" class="interlanguage-link-target"><span>閩東語 / Mìng-dĕ̤ng-ngṳ̄</span></a></li><li class="interlanguage-link interwiki-nl badge-Q70894304 mw-list-item" title=""><a href="https://nl.wikipedia.org/wiki/Tweeling_(biologie)" title="Tweeling (biologie) – Dutch" lang="nl" hreflang="nl" data-title="Tweeling (biologie)" data-language-autonym="Nederlands" data-language-local-name="Dutch" class="interlanguage-link-target"><span>Nederlands</span></a></li><li class="interlanguage-link interwiki-ja mw-list-item"><a href="https://ja.wikipedia.org/wiki/%E5%8F%8C%E7%94%9F%E5%85%90" title="双生児 – Japanese" lang="ja" hreflang="ja" data-title="双生児" data-language-autonym="日本語" data-language-local-name="Japanese" class="interlanguage-link-target"><span>日本語</span></a></li><li class="interlanguage-link interwiki-no badge-Q70894304 mw-list-item" title=""><a href="https://no.wikipedia.org/wiki/Tvilling" title="Tvilling – Norwegian Bokmål" lang="nb" hreflang="nb" data-title="Tvilling" data-language-autonym="Norsk bokmål" data-language-local-name="Norwegian Bokmål" class="interlanguage-link-target"><span>Norsk bokmål</span></a></li><li class="interlanguage-link interwiki-nn mw-list-item"><a href="https://nn.wikipedia.org/wiki/Tvilling" title="Tvilling – Norwegian Nynorsk" lang="nn" hreflang="nn" data-title="Tvilling" data-language-autonym="Norsk nynorsk" data-language-local-name="Norwegian Nynorsk" class="interlanguage-link-target"><span>Norsk nynorsk</span></a></li><li class="interlanguage-link interwiki-nrm mw-list-item"><a href="https://nrm.wikipedia.org/wiki/Gemiaux_(fratrie)" title="Gemiaux (fratrie) – Norman" lang="nrf" hreflang="nrf" data-title="Gemiaux (fratrie)" data-language-autonym="Nouormand" data-language-local-name="Norman" class="interlanguage-link-target"><span>Nouormand</span></a></li><li class="interlanguage-link interwiki-oc mw-list-item"><a href="https://oc.wikipedia.org/wiki/Besson" title="Besson – Occitan" lang="oc" hreflang="oc" data-title="Besson" data-language-autonym="Occitan" data-language-local-name="Occitan" class="interlanguage-link-target"><span>Occitan</span></a></li><li class="interlanguage-link interwiki-uz mw-list-item"><a href="https://uz.wikipedia.org/wiki/Egizaklar" title="Egizaklar – Uzbek" lang="uz" hreflang="uz" data-title="Egizaklar" data-language-autonym="Oʻzbekcha / ўзбекча" data-language-local-name="Uzbek" class="interlanguage-link-target"><span>Oʻzbekcha / ўзбекча</span></a></li><li class="interlanguage-link interwiki-pa mw-list-item"><a href="https://pa.wikipedia.org/wiki/%E0%A8%9C%E0%A9%8C%E0%A9%9C%E0%A9%87" title="ਜੌੜੇ – Punjabi" lang="pa" hreflang="pa" data-title="ਜੌੜੇ" data-language-autonym="ਪੰਜਾਬੀ" data-language-local-name="Punjabi" class="interlanguage-link-target"><span>ਪੰਜਾਬੀ</span></a></li><li class="interlanguage-link interwiki-ps mw-list-item"><a href="https://ps.wikipedia.org/wiki/%D8%BA%D8%A8%D8%B1%DA%AB%D9%88%D9%86%D9%8A" title="غبرګوني – Pashto" lang="ps" hreflang="ps" data-title="غبرګوني" data-language-autonym="پښتو" data-language-local-name="Pashto" class="interlanguage-link-target"><span>پښتو</span></a></li><li class="interlanguage-link interwiki-pl mw-list-item"><a href="https://pl.wikipedia.org/wiki/Ci%C4%85%C5%BCa_bli%C5%BAniacza" title="Ciąża bliźniacza – Polish" lang="pl" hreflang="pl" data-title="Ciąża bliźniacza" data-language-autonym="Polski" data-language-local-name="Polish" class="interlanguage-link-target"><span>Polski</span></a></li><li class="interlanguage-link interwiki-pt mw-list-item"><a href="https://pt.wikipedia.org/wiki/G%C3%AAmeos" title="Gêmeos – Portuguese" lang="pt" hreflang="pt" data-title="Gêmeos" data-language-autonym="Português" data-language-local-name="Portuguese" class="interlanguage-link-target"><span>Português</span></a></li><li class="interlanguage-link interwiki-ro mw-list-item"><a href="https://ro.wikipedia.org/wiki/Geam%C4%83n" title="Geamăn – Romanian" lang="ro" hreflang="ro" data-title="Geamăn" data-language-autonym="Română" data-language-local-name="Romanian" class="interlanguage-link-target"><span>Română</span></a></li><li class="interlanguage-link interwiki-qu mw-list-item"><a href="https://qu.wikipedia.org/wiki/T%27ira" title="T'ira – Quechua" lang="qu" hreflang="qu" data-title="T'ira" data-language-autonym="Runa Simi" data-language-local-name="Quechua" class="interlanguage-link-target"><span>Runa Simi</span></a></li><li class="interlanguage-link interwiki-ru mw-list-item"><a href="https://ru.wikipedia.org/wiki/%D0%91%D0%BB%D0%B8%D0%B7%D0%BD%D0%B5%D1%86%D1%8B" title="Близнецы – Russian" lang="ru" hreflang="ru" data-title="Близнецы" data-language-autonym="Русский" data-language-local-name="Russian" class="interlanguage-link-target"><span>Русский</span></a></li><li class="interlanguage-link interwiki-sq mw-list-item"><a href="https://sq.wikipedia.org/wiki/Binjak%C3%ABt" title="Binjakët – Albanian" lang="sq" hreflang="sq" data-title="Binjakët" data-language-autonym="Shqip" data-language-local-name="Albanian" class="interlanguage-link-target"><span>Shqip</span></a></li><li class="interlanguage-link interwiki-scn mw-list-item"><a href="https://scn.wikipedia.org/wiki/Giameddu" title="Giameddu – Sicilian" lang="scn" hreflang="scn" data-title="Giameddu" data-language-autonym="Sicilianu" data-language-local-name="Sicilian" class="interlanguage-link-target"><span>Sicilianu</span></a></li><li class="interlanguage-link interwiki-simple mw-list-item"><a href="https://simple.wikipedia.org/wiki/Twin" title="Twin – Simple English" lang="en-simple" hreflang="en-simple" data-title="Twin" data-language-autonym="Simple English" data-language-local-name="Simple English" class="interlanguage-link-target"><span>Simple English</span></a></li><li class="interlanguage-link interwiki-sd mw-list-item"><a href="https://sd.wikipedia.org/wiki/%D8%AC%D8%A7%DA%99%D8%A7" title="جاڙا – Sindhi" lang="sd" hreflang="sd" data-title="جاڙا" data-language-autonym="سنڌي" 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class="mw-content-ltr mw-parser-output" lang="en" dir="ltr"><div class="shortdescription nomobile noexcerpt noprint searchaux" style="display:none">One of two offspring produced by the same pregnancy</div> <style data-mw-deduplicate="TemplateStyles:r1236090951">.mw-parser-output .hatnote{font-style:italic}.mw-parser-output div.hatnote{padding-left:1.6em;margin-bottom:0.5em}.mw-parser-output .hatnote i{font-style:normal}.mw-parser-output .hatnote+link+.hatnote{margin-top:-0.5em}@media print{body.ns-0 .mw-parser-output .hatnote{display:none!important}}</style><div role="note" class="hatnote navigation-not-searchable">For other uses, see <a href="/wiki/Twin_(disambiguation)" class="mw-disambig" title="Twin (disambiguation)">Twin (disambiguation)</a> and <a href="/wiki/Twins_(disambiguation)" class="mw-disambig" title="Twins (disambiguation)">Twins (disambiguation)</a>.</div> <figure class="mw-default-size" typeof="mw:File/Thumb"><a href="/wiki/File:Mauch_Twins.jpg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/2/2b/Mauch_Twins.jpg/220px-Mauch_Twins.jpg" decoding="async" width="220" height="261" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/2/2b/Mauch_Twins.jpg/330px-Mauch_Twins.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/2/2b/Mauch_Twins.jpg/440px-Mauch_Twins.jpg 2x" data-file-width="523" data-file-height="620" /></a><figcaption>Identical twins can be difficult to visually distinguish when young, as demonstrated by brothers <a href="/wiki/Billy_and_Bobby_Mauch" title="Billy and Bobby Mauch">Billy and Bobby Mauch</a>. </figcaption></figure> <p><b>Twins</b> are two <a href="/wiki/Offspring" title="Offspring">offspring</a> produced by the same <a href="/wiki/Pregnancy" title="Pregnancy">pregnancy</a>.<sup id="cite_ref-MedicineNet_1-0" class="reference"><a href="#cite_note-MedicineNet-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> Twins can be either <i>monozygotic</i> ('identical'), meaning that they develop from one <a href="/wiki/Zygote" title="Zygote">zygote</a>, which splits and forms two <a href="/wiki/Embryo" title="Embryo">embryos</a>, or <i>dizygotic</i> ('non-identical' or 'fraternal'), meaning that each twin develops from a separate egg and each egg is fertilized by its own sperm cell.<sup id="cite_ref-2" class="reference"><a href="#cite_note-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> Since identical twins develop from one zygote, they will share the same sex, while fraternal twins may or may not. In very rare cases fraternal (and <a href="#Half-identical_twins">semi- or half-identical</a>) twins can have the same mother and different fathers (<a href="/wiki/Superfecundation" title="Superfecundation">heteropaternal superfecundation</a>). </p><p>In contrast, a <a href="/wiki/Fetus" title="Fetus">fetus</a> that develops alone in the <a href="/wiki/Uterus" title="Uterus">womb</a> (the much more common case in humans) is called a <i>singleton</i>, and the general term for one offspring of a <a href="/wiki/Multiple_birth" title="Multiple birth">multiple birth</a> is a <i>multiple</i>.<sup id="cite_ref-3" class="reference"><a href="#cite_note-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup> Unrelated <a href="/wiki/Look-alike" title="Look-alike">look-alikes</a> whose resemblance parallels that of twins are referred to as <a href="/wiki/Doppelg%C3%A4nger" title="Doppelgänger">doppelgänger</a>.<sup id="cite_ref-4" class="reference"><a href="#cite_note-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup> </p> <style data-mw-deduplicate="TemplateStyles:r886046785">.mw-parser-output .toclimit-2 .toclevel-1 ul,.mw-parser-output .toclimit-3 .toclevel-2 ul,.mw-parser-output .toclimit-4 .toclevel-3 ul,.mw-parser-output .toclimit-5 .toclevel-4 ul,.mw-parser-output .toclimit-6 .toclevel-5 ul,.mw-parser-output .toclimit-7 .toclevel-6 ul{display:none}</style><div class="toclimit-3"><meta property="mw:PageProp/toc" /></div> <div class="mw-heading mw-heading2"><h2 id="Statistics">Statistics</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Twin&action=edit&section=1" title="Edit section: Statistics"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>The human twin birth rate in the United States rose 76% from 1980 through 2009, from 9.4 to 16.7 twin sets (18.8 to 33.3 twins) per 1,000 births.<sup id="cite_ref-5" class="reference"><a href="#cite_note-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup> The <a href="/wiki/Yoruba_people" title="Yoruba people">Yoruba people</a> have the highest rate of twinning in the world, at 45–50 twin sets (90–100 twins) per 1,000 live births,<sup id="cite_ref-6" class="reference"><a href="#cite_note-6"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-7" class="reference"><a href="#cite_note-7"><span class="cite-bracket">[</span>7<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-8" class="reference"><a href="#cite_note-8"><span class="cite-bracket">[</span>8<span class="cite-bracket">]</span></a></sup> possibly because of high consumption of a specific type of <a href="/wiki/Yam_(vegetable)" title="Yam (vegetable)">yam</a> containing a natural <a href="/wiki/Phytoestrogen" title="Phytoestrogen">phytoestrogen</a> which may stimulate the <a href="/wiki/Ovary" title="Ovary">ovarys</a> to release an egg from each side.<sup id="cite_ref-9" class="reference"><a href="#cite_note-9"><span class="cite-bracket">[</span>9<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-10" class="reference"><a href="#cite_note-10"><span class="cite-bracket">[</span>10<span class="cite-bracket">]</span></a></sup> In <a href="/wiki/Central_Africa" title="Central Africa">Central Africa</a>, there are 18–30 twin sets (or 36–60 twins) per 1,000 live births.<sup id="cite_ref-Smits_11-0" class="reference"><a href="#cite_note-Smits-11"><span class="cite-bracket">[</span>11<span class="cite-bracket">]</span></a></sup> In <a href="/wiki/South_America" title="South America">South America</a>, <a href="/wiki/South_Asia" title="South Asia">South Asia</a>, and <a href="/wiki/Southeast_Asia" title="Southeast Asia">Southeast Asia</a>, the lowest rates are found; only 6 to 9 twin sets per 1,000 live births. North America and Europe have intermediate rates of 9 to 16 twin sets per 1,000 live births.<sup id="cite_ref-Smits_11-1" class="reference"><a href="#cite_note-Smits-11"><span class="cite-bracket">[</span>11<span class="cite-bracket">]</span></a></sup> </p><p>Multiple pregnancies are much less likely to carry to full term than single births, with twin pregnancies lasting on average 37 weeks, three weeks less than full term.<sup id="cite_ref-12" class="reference"><a href="#cite_note-12"><span class="cite-bracket">[</span>12<span class="cite-bracket">]</span></a></sup> Women who have a family history of fraternal twins have a higher chance of producing fraternal twins themselves, as there is a genetically linked tendency to hyper-<a href="/wiki/Ovulation" title="Ovulation">ovulate</a>. There is no known genetic link for identical twinning.<sup id="cite_ref-13" class="reference"><a href="#cite_note-13"><span class="cite-bracket">[</span>13<span class="cite-bracket">]</span></a></sup> Other factors that increase the odds of having fraternal twins include maternal age, fertility drugs and other fertility treatments, nutrition, and prior births.<sup id="cite_ref-14" class="reference"><a href="#cite_note-14"><span class="cite-bracket">[</span>14<span class="cite-bracket">]</span></a></sup> Some women intentionally turn to fertility drugs in order to conceive twins.<sup id="cite_ref-15" class="reference"><a href="#cite_note-15"><span class="cite-bracket">[</span>15<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-16" class="reference"><a href="#cite_note-16"><span class="cite-bracket">[</span>16<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading2"><h2 id="Types_and_zygosity">Types and zygosity</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Twin&action=edit&section=2" title="Edit section: Types and zygosity"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>The vast majority of twins are either dizygotic (fraternal) or monozygotic (identical). In humans dizygotic twins occur more often than monozygotic twins.<sup id="cite_ref-17" class="reference"><a href="#cite_note-17"><span class="cite-bracket">[</span>17<span class="cite-bracket">]</span></a></sup> Less common variants are discussed further down the article. </p><p>Fraternal twins can be any of the following: </p> <ul><li>Female–female twins: Sometimes called sororal twins (25%).</li> <li>Male–male twins: Sometimes called fraternal (unrelated to zygosity) twins (25%).</li> <li>Female-male twins: This is the most common pairing (50%), encompassing both "female-male" (25%) and "male-female" (25%) twins.</li></ul> <p>Among non-twin births, male singletons are slightly (about five percent) more common than female singletons. The rates for singletons vary slightly by country. For example, the sex ratio of birth in the US is 1.05 males/female,<sup id="cite_ref-18" class="reference"><a href="#cite_note-18"><span class="cite-bracket">[</span>18<span class="cite-bracket">]</span></a></sup> while it is 1.07 males/female in Italy.<sup id="cite_ref-19" class="reference"><a href="#cite_note-19"><span class="cite-bracket">[</span>19<span class="cite-bracket">]</span></a></sup> However, males are also more susceptible than females to die <i>in utero</i>, and since the death rate <i>in utero</i> is higher for twins, it leads to female twins being more common than male twins.<sup id="cite_ref-WSTR_20-0" class="reference"><a href="#cite_note-WSTR-20"><span class="cite-bracket">[</span>20<span class="cite-bracket">]</span></a></sup> </p><p><a href="/wiki/Zygosity" title="Zygosity">Zygosity</a> is the degree of identity in the <a href="/wiki/Genome" title="Genome">genome</a> of twins. </p> <div class="mw-heading mw-heading3"><h3 id="Dizygotic_(fraternal)_twins"><span id="Dizygotic_.28fraternal.29_twins"></span><span class="anchor" id="Fraternal"></span><span class="anchor" id="Fraternal_twins"></span><span class="anchor" id="Dizygotic"></span><span class="anchor" id="Fraternal_(dizygotic)_twins"></span>Dizygotic (fraternal) twins</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Twin&action=edit&section=3" title="Edit section: Dizygotic (fraternal) twins"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <figure class="mw-default-size mw-halign-right" typeof="mw:File/Thumb"><a href="/wiki/File:Red-haired_Siblings.jpg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/e/e4/Red-haired_Siblings.jpg/220px-Red-haired_Siblings.jpg" decoding="async" width="220" height="176" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/e/e4/Red-haired_Siblings.jpg/330px-Red-haired_Siblings.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/e/e4/Red-haired_Siblings.jpg/440px-Red-haired_Siblings.jpg 2x" data-file-width="1200" data-file-height="960" /></a><figcaption>Adult fraternal twins</figcaption></figure> <figure class="mw-default-size" typeof="mw:File/Thumb"><a href="/wiki/File:Fraternal_twin_brothers.jpg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/f/fe/Fraternal_twin_brothers.jpg/220px-Fraternal_twin_brothers.jpg" decoding="async" width="220" height="165" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/f/fe/Fraternal_twin_brothers.jpg 1.5x" data-file-width="302" data-file-height="226" /></a><figcaption>Fraternal twin brothers as young babies</figcaption></figure> <p><b>Dizygotic</b> (<b>DZ</b>) or <b>fraternal</b> <b>twins</b> (also referred to as "non-identical twins", "dissimilar twins", "biovular twins", and, informally in the case of females, "sororal twins") usually occur when two <a href="/wiki/Fertilisation" title="Fertilisation">fertilized</a> eggs are implanted in the uterus wall at the same time. When two eggs are independently fertilized by two different <a href="/wiki/Spermatozoon" title="Spermatozoon">sperm cells</a>, fraternal twins result. The two eggs, or <i>ova</i>, form two <a href="/wiki/Zygote" title="Zygote">zygotes</a>, hence the terms <i>dizygotic</i> and <i>biovular</i>. Fraternal twins are, essentially, two ordinary <a href="/wiki/Sibling" title="Sibling">siblings</a> who happen to develop in the womb together and who are born at the same time, since they arise from two separate eggs fertilized by two separate <a href="/wiki/Sperm" title="Sperm">sperm</a>, just like ordinary siblings. This is the most common type of twin.<sup id="cite_ref-multi_21-0" class="reference"><a href="#cite_note-multi-21"><span class="cite-bracket">[</span>21<span class="cite-bracket">]</span></a></sup> </p><p>Dizygotic twins, like any other siblings, will practically always have different sequences on each chromosome, due to <a href="/wiki/Chromosomal_crossover" title="Chromosomal crossover">chromosomal crossover</a> during <a href="/wiki/Meiosis" title="Meiosis">meiosis</a>. Dizygotic twins share on average 50 percent of each other's genes, the same as siblings that are conceived and born at different times. Like any other <a href="/wiki/Sibling" title="Sibling">siblings</a>, dizygotic twins <a href="/wiki/Family_resemblance_(anthropology)" title="Family resemblance (anthropology)">may look similar</a>, particularly as they are the same age. However, dizygotic twins may also look very different from each other (for example, be of opposite sexes). </p><p>Studies show that there is a genetic proclivity for dizygotic twinning. However, it is only the mother who has any effect on the chances of having such twins; there is no known mechanism for a father to cause the release of more than one <a href="/wiki/Ovum" class="mw-redirect" title="Ovum">ovum</a>. Dizygotic twinning ranges from six per thousand births in Japan (similar to the rate of monozygotic twins) to 14 and more per thousand in some African countries.<sup id="cite_ref-22" class="reference"><a href="#cite_note-22"><span class="cite-bracket">[</span>22<span class="cite-bracket">]</span></a></sup> </p><p>Dizygotic twins are also more common for older mothers, with twinning rates doubling in mothers over the age of 35.<sup id="cite_ref-Bortolus_23-0" class="reference"><a href="#cite_note-Bortolus-23"><span class="cite-bracket">[</span>23<span class="cite-bracket">]</span></a></sup> With the advent of technologies and techniques to assist women in getting pregnant, the rate of fraternals has increased markedly.<sup class="noprint Inline-Template Template-Fact" style="white-space:nowrap;">[<i><a href="/wiki/Wikipedia:Citation_needed" title="Wikipedia:Citation needed"><span title="This claim needs references to reliable sources. (August 2022)">citation needed</span></a></i>]</sup> </p> <div class="mw-heading mw-heading3"><h3 id="Monozygotic_(identical)_twins"><span id="Monozygotic_.28identical.29_twins"></span><span class="anchor" id="IdenticalTwins"></span><span class="anchor" id="Identical_twins"></span><span class="anchor" id="Identical_(monozygotic)_twins"></span>Monozygotic (identical) twins</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Twin&action=edit&section=4" title="Edit section: Monozygotic (identical) twins"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p><b>Monozygotic</b> (<b>MZ</b>) or <b>identical</b> <b>twins</b> occur when a single <a href="/wiki/Ovum" class="mw-redirect" title="Ovum">egg</a> is fertilized to form one <a href="/wiki/Zygote" title="Zygote">zygote</a> (hence, "monozygotic") which then divides into two separate <a href="/wiki/Embryo" title="Embryo">embryos</a>. The chances of having identical twins is relatively rare – around 3 or 4 in every 1,000 births.<sup id="cite_ref-mlp_24-0" class="reference"><a href="#cite_note-mlp-24"><span class="cite-bracket">[</span>24<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading4"><h4 id="Mechanism">Mechanism</h4><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Twin&action=edit&section=5" title="Edit section: Mechanism"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Regarding spontaneous or <i>natural</i> monozygotic twinning, a 2007 theory related to in vitro fertilization (IVF) proposes that monozygotic twins may be formed when a <a href="/wiki/Blastocyst" title="Blastocyst">blastocyst</a> contains two inner cell masses (ICM), each of which will lead to a separate fetus, rather than by the embryo splitting while hatching from the <a href="/wiki/Zona_pellucida" title="Zona pellucida">zona pellucida</a> (the gelatinous protective coating around the blastocyst).<sup id="cite_ref-25" class="reference"><a href="#cite_note-25"><span class="cite-bracket">[</span>25<span class="cite-bracket">]</span></a></sup> </p><p>Monozygotic twins may also be created artificially by embryo splitting. It can be used as an expansion of <a href="/wiki/In_vitro_fertilization" class="mw-redirect" title="In vitro fertilization">in vitro fertilization</a> (IVF) to increase the number of available embryos for <a href="/wiki/Embryo_transfer" title="Embryo transfer">embryo transfer</a>.<sup id="cite_ref-26" class="reference"><a href="#cite_note-26"><span class="cite-bracket">[</span>26<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading4"><h4 id="Incidence">Incidence</h4><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Twin&action=edit&section=6" title="Edit section: Incidence"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Monozygotic twinning occurs in birthing at a rate of about 3 in every 1000 deliveries worldwide.<sup id="cite_ref-Holladay_2001_27-0" class="reference"><a href="#cite_note-Holladay_2001-27"><span class="cite-bracket">[</span>27<span class="cite-bracket">]</span></a></sup> </p><p>The likelihood of a single fertilization resulting in monozygotic twins is uniformly distributed in all populations around the world.<sup id="cite_ref-Bortolus_23-1" class="reference"><a href="#cite_note-Bortolus-23"><span class="cite-bracket">[</span>23<span class="cite-bracket">]</span></a></sup> This is in marked contrast to dizygotic twinning, which ranges from about six per thousand births in Japan (almost similar to the rate of identical twins, which is around 4–5) to 15 and more per thousand in some parts of India<sup id="cite_ref-28" class="reference"><a href="#cite_note-28"><span class="cite-bracket">[</span>28<span class="cite-bracket">]</span></a></sup> and up to over 20 in some Central African countries.<sup id="cite_ref-Smits_11-2" class="reference"><a href="#cite_note-Smits-11"><span class="cite-bracket">[</span>11<span class="cite-bracket">]</span></a></sup> The exact cause for the splitting of a zygote or embryo is unknown. </p><p>IVF techniques are more likely to create dizygotic twins. For IVF deliveries, there are nearly 21 pairs of twins for every 1,000.<sup id="cite_ref-29" class="reference"><a href="#cite_note-29"><span class="cite-bracket">[</span>29<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading4"><h4 id="Genetic_and_epigenetic_similarity">Genetic and epigenetic similarity</h4><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Twin&action=edit&section=7" title="Edit section: Genetic and epigenetic similarity"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <figure class="mw-default-size" typeof="mw:File/Thumb"><a href="/wiki/File:Identical-fraternal-sperm-egg.svg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/9/98/Identical-fraternal-sperm-egg.svg/300px-Identical-fraternal-sperm-egg.svg.png" decoding="async" width="300" height="206" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/9/98/Identical-fraternal-sperm-egg.svg/450px-Identical-fraternal-sperm-egg.svg.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/9/98/Identical-fraternal-sperm-egg.svg/600px-Identical-fraternal-sperm-egg.svg.png 2x" data-file-width="500" data-file-height="344" /></a><figcaption>Comparison of zygote development in monozygotic and dizygotic twins. In the uterus, a majority of monozygotic twins (60–70%) share the same <a href="/wiki/Placenta" title="Placenta">placenta</a> but have separate <a href="/wiki/Amniotic_sac" title="Amniotic sac">amniotic sacs</a>. In 18–30% of monozygotic twins each fetus has a separate placenta and a separate amniotic sac. A small number (1–2%) of monozygotic twins share the same placenta and amniotic sac. Fraternal twins each have their own placenta and own amniotic sac.</figcaption></figure> <p>Monozygotic twins are genetically nearly identical and they are the same chromosomal sex unless there has been a mutation during development. The children of monozygotic twins test genetically as half-siblings (or full siblings, if a pair of monozygotic twins reproduces with another pair or with the same person), rather than first cousins. Identical twins do not have the same <a href="/wiki/Fingerprint" title="Fingerprint">fingerprints</a> however, because even within the confines of the womb, the fetuses touch different parts of their environment, giving rise to small variations in their corresponding prints and thus making them unique.<sup id="cite_ref-30" class="reference"><a href="#cite_note-30"><span class="cite-bracket">[</span>30<span class="cite-bracket">]</span></a></sup> </p><p>Monozygotic twins always have the same <a href="/wiki/Genotype" title="Genotype">genotype</a>. Normally due to an environmental factor or the deactivation of different X chromosomes in female monozygotic twins, and in some extremely rare cases, due to <a href="/wiki/Aneuploidy" title="Aneuploidy">aneuploidy</a>, twins may express different sexual <a href="/wiki/Phenotype" title="Phenotype">phenotypes</a>, normally from an XXY <a href="/wiki/Klinefelter_syndrome" title="Klinefelter syndrome">Klinefelter syndrome</a> zygote splitting unevenly.<sup id="cite_ref-31" class="reference"><a href="#cite_note-31"><span class="cite-bracket">[</span>31<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-32" class="reference"><a href="#cite_note-32"><span class="cite-bracket">[</span>32<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Schmid2000_33-0" class="reference"><a href="#cite_note-Schmid2000-33"><span class="cite-bracket">[</span>33<span class="cite-bracket">]</span></a></sup> </p><p>Monozygotic twins, although genetically very similar, are not genetically exactly the same. The DNA in white blood cells of 66 pairs of monozygotic twins was analyzed for 506,786 <a href="/wiki/Single-nucleotide_polymorphism" title="Single-nucleotide polymorphism">single-nucleotide polymorphisms</a> known to occur in human populations. Polymorphisms appeared in 2 of the 33 million comparisons, leading the researchers to extrapolate that the blood cells of monozygotic twins may have on the order of one DNA-sequence difference for every 12 million nucleotides, which would imply hundreds of differences across the entire genome.<sup id="cite_ref-pmid24123875_34-0" class="reference"><a href="#cite_note-pmid24123875-34"><span class="cite-bracket">[</span>34<span class="cite-bracket">]</span></a></sup> The mutations producing the differences detected in this study would have occurred during embryonic cell-division (after the point of fertilization). If they occur early in fetal development, they will be present in a very large proportion of body cells.<sup class="noprint Inline-Template Template-Fact" style="white-space:nowrap;">[<i><a href="/wiki/Wikipedia:Citation_needed" title="Wikipedia:Citation needed"><span title="This claim needs references to reliable sources. (February 2024)">citation needed</span></a></i>]</sup> </p> <figure class="mw-default-size" typeof="mw:File/Thumb"><a href="/wiki/File:Mark_and_Scott_Kelly_at_the_Johnson_Space_Center,_Houston_Texas.jpg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/9/97/Mark_and_Scott_Kelly_at_the_Johnson_Space_Center%2C_Houston_Texas.jpg/250px-Mark_and_Scott_Kelly_at_the_Johnson_Space_Center%2C_Houston_Texas.jpg" decoding="async" width="250" height="167" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/9/97/Mark_and_Scott_Kelly_at_the_Johnson_Space_Center%2C_Houston_Texas.jpg/375px-Mark_and_Scott_Kelly_at_the_Johnson_Space_Center%2C_Houston_Texas.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/9/97/Mark_and_Scott_Kelly_at_the_Johnson_Space_Center%2C_Houston_Texas.jpg/500px-Mark_and_Scott_Kelly_at_the_Johnson_Space_Center%2C_Houston_Texas.jpg 2x" data-file-width="7360" data-file-height="4912" /></a><figcaption>Despite being genetically identical, twins <a href="/wiki/Mark_Kelly" title="Mark Kelly">Mark</a> and <a href="/wiki/Scott_Kelly_(astronaut)" title="Scott Kelly (astronaut)">Scott Kelly</a> have become distinguishable from each other by adulthood. </figcaption></figure> <p>Another cause of difference between monozygotic twins is <a href="/wiki/Epigenetics" title="Epigenetics">epigenetic modification</a>, caused by differing environmental influences throughout their lives. Epigenetics refers to the level of activity of any particular gene. A gene may become switched on, switched off, or could become partially switched on or off in an individual. This epigenetic modification is triggered by environmental events. Monozygotic twins can have markedly different epigenetic profiles. A study of 80 pairs of monozygotic twins ranging in age from three to 74 showed that the youngest twins have relatively few <a href="/wiki/Epigenetics" title="Epigenetics">epigenetic</a> differences. The number of epigenetic differences increases with age. Fifty-year-old twins had over three times the epigenetic difference of three-year-old twins. Twins who had spent their lives apart (such as those adopted by two different sets of parents at birth) had the greatest difference.<sup id="cite_ref-35" class="reference"><a href="#cite_note-35"><span class="cite-bracket">[</span>35<span class="cite-bracket">]</span></a></sup> However, certain characteristics become more alike as twins age, such as IQ and personality.<sup id="cite_ref-36" class="reference"><a href="#cite_note-36"><span class="cite-bracket">[</span>36<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-37" class="reference"><a href="#cite_note-37"><span class="cite-bracket">[</span>37<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-38" class="reference"><a href="#cite_note-38"><span class="cite-bracket">[</span>38<span class="cite-bracket">]</span></a></sup> </p><p>In January 2021, new research from a team of researchers in <a href="/wiki/Iceland" title="Iceland">Iceland</a> was published in the journal <i><a href="/wiki/Nature_Genetics" title="Nature Genetics">Nature Genetics</a></i> suggesting that identical twins may not be quite as identical as previously thought.<sup id="cite_ref-39" class="reference"><a href="#cite_note-39"><span class="cite-bracket">[</span>39<span class="cite-bracket">]</span></a></sup> The four-year study of monozygotic (identical) twins and their extended families revealed that these twins have genetic differences that begin in the early stages of embryonic development.<sup id="cite_ref-40" class="reference"><a href="#cite_note-40"><span class="cite-bracket">[</span>40<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading3"><h3 id="Polar_body_and_semi-identical_twins">Polar body and semi-identical twins</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Twin&action=edit&section=8" title="Edit section: Polar body and semi-identical twins"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>A 1981 study of a deceased <a href="/wiki/Triploid_syndrome" title="Triploid syndrome">XXX</a> twin fetus <a href="/wiki/Parasitic_twin" title="Parasitic twin">without a heart</a> showed that although its fetal development suggested that it was an identical twin, as it shared a placenta with its healthy twin, tests revealed that it was probably a <a href="/wiki/Polar_body" title="Polar body">polar body</a> twin. The authors were unable to predict whether a healthy fetus could result from a polar body twinning.<sup id="cite_ref-41" class="reference"><a href="#cite_note-41"><span class="cite-bracket">[</span>41<span class="cite-bracket">]</span></a></sup> However, a study in 2012 found that it is possible for a polar body to result in a healthy fetus.<sup id="cite_ref-42" class="reference"><a href="#cite_note-42"><span class="cite-bracket">[</span>42<span class="cite-bracket">]</span></a></sup> </p><p>In 2003, a study argued that many cases of <a href="/wiki/Triploid_syndrome" title="Triploid syndrome">triploidy</a> arise from sesquizygotic (semi-identical) twinning which happens when a single egg is fertilized by two sperm and splits the three sets of chromosomes into two separate cell sets.<sup id="cite_ref-43" class="reference"><a href="#cite_note-43"><span class="cite-bracket">[</span>43<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-44" class="reference"><a href="#cite_note-44"><span class="cite-bracket">[</span>44<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading2"><h2 id="Degree_of_separation">Degree of separation</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Twin&action=edit&section=9" title="Edit section: Degree of separation"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p><span class="anchor" id="Chorionicity"></span> </p> <figure class="mw-default-size" typeof="mw:File/Thumb"><a href="/wiki/File:Placentation.svg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/9/93/Placentation.svg/220px-Placentation.svg.png" decoding="async" width="220" height="351" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/9/93/Placentation.svg/330px-Placentation.svg.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/9/93/Placentation.svg/440px-Placentation.svg.png 2x" data-file-width="472" data-file-height="753" /></a><figcaption>Various types of chorionicity and amniosity (how the baby's sac looks) in monozygotic (one egg/identical) twins as a result of when the fertilized egg divides</figcaption></figure> <p>The degree of separation of the twins in utero depends on if and when they split into two zygotes. Dizygotic twins were always two zygotes. Monozygotic twins split into two zygotes at some time very early in the pregnancy. The timing of this separation determines the chorionicity (the number of placentae) and amniocity (the number of sacs) of the pregnancy. Dichorionic twins either never divided (i.e.: were dizygotic) or they divided within the first 4 days. Monoamnionic twins divide after the first week.<sup class="noprint Inline-Template Template-Fact" style="white-space:nowrap;">[<i><a href="/wiki/Wikipedia:Citation_needed" title="Wikipedia:Citation needed"><span title="This claim needs references to reliable sources. (February 2024)">citation needed</span></a></i>]</sup> </p><p>In very rare cases, twins become <a href="/wiki/Conjoined_twins" title="Conjoined twins">conjoined twins</a>. Non-conjoined monozygotic twins form up to day 14 of embryonic development, but when twinning occurs after 14 days, the twins will likely be conjoined.<sup id="cite_ref-45" class="reference"><a href="#cite_note-45"><span class="cite-bracket">[</span>45<span class="cite-bracket">]</span></a></sup> Furthermore, there can be various degrees of shared environment of twins in the womb, potentially leading to <a href="/wiki/Complications_of_pregnancy" title="Complications of pregnancy">pregnancy complications</a>.<sup class="noprint Inline-Template Template-Fact" style="white-space:nowrap;">[<i><a href="/wiki/Wikipedia:Citation_needed" title="Wikipedia:Citation needed"><span title="This claim needs references to reliable sources. (February 2024)">citation needed</span></a></i>]</sup> </p><p>It is a common misconception that two <a href="/wiki/Placenta" title="Placenta">placentas</a> automatically implies dizygotic twins, but if monozygotic twins separate early enough, the arrangement of sacs and placentas in utero is in fact indistinguishable from that of dizygotic twins. </p> <table class="wikitable"> <tbody><tr> <th>Type </th> <th>Description </th> <th>Day </th></tr> <tr> <td>Dichorionic–Diamniotic </td> <td>Normally, twins have two separate (di- being a <a href="/wiki/Numerical_prefix" class="mw-redirect" title="Numerical prefix">numerical prefix</a> for two) <a href="/wiki/Chorion" title="Chorion">chorions</a> and <a href="/wiki/Amniotic_sac" title="Amniotic sac">amniotic sacs</a>, termed Dichorionic–Diamniotic or "DiDi". It occurs in almost all cases of dizygotic twins (except in very rare cases of fusion between their blastocysts<sup id="cite_ref-Shulman_46-0" class="reference"><a href="#cite_note-Shulman-46"><span class="cite-bracket">[</span>46<span class="cite-bracket">]</span></a></sup>) and in 18–36%<sup id="cite_ref-splitstats_47-0" class="reference"><a href="#cite_note-splitstats-47"><span class="cite-bracket">[</span>47<span class="cite-bracket">]</span></a></sup> (or around 25%<sup id="cite_ref-Shulman_46-1" class="reference"><a href="#cite_note-Shulman-46"><span class="cite-bracket">[</span>46<span class="cite-bracket">]</span></a></sup>) of <a href="/wiki/Monozygotic" class="mw-redirect" title="Monozygotic">monozygotic</a> (identical) twins. <p>DiDi twins have the lowest mortality risk at about 9 percent, although that is still significantly higher than that of singletons.<sup id="cite_ref-48" class="reference"><a href="#cite_note-48"><span class="cite-bracket">[</span>48<span class="cite-bracket">]</span></a></sup> </p> </td> <td>Dichorionic-Diamniotic twins form when splitting takes place by the third day after <a href="/wiki/Fertilisation" title="Fertilisation">fertilisation</a>.<sup id="cite_ref-Shulman_46-2" class="reference"><a href="#cite_note-Shulman-46"><span class="cite-bracket">[</span>46<span class="cite-bracket">]</span></a></sup> </td></tr> <tr> <td><a href="/wiki/Monochorionic_twins" title="Monochorionic twins">Monochorionic–Diamniotic</a> </td> <td>Monochorionic twins share the same <a href="/wiki/Placenta" title="Placenta">placenta</a>. <p>Monochorionic twins generally have two <a href="/wiki/Amniotic_sac" title="Amniotic sac">amniotic sacs</a> (called Monochorionic–Diamniotic "MoDi"), which occurs in 60–70% of the pregnancies with monozygotic twins,<sup id="cite_ref-splitstats_47-1" class="reference"><a href="#cite_note-splitstats-47"><span class="cite-bracket">[</span>47<span class="cite-bracket">]</span></a></sup> and in 0.3% of all pregnancies.<sup id="cite_ref-cordero_49-0" class="reference"><a href="#cite_note-cordero-49"><span class="cite-bracket">[</span>49<span class="cite-bracket">]</span></a></sup> Monochorionic-Diamniotic twins are <i>almost always</i> monozygotic, with a few exceptions where the <a href="/wiki/Blastocyst" title="Blastocyst">blastocysts</a> have fused.<sup id="cite_ref-Shulman_46-3" class="reference"><a href="#cite_note-Shulman-46"><span class="cite-bracket">[</span>46<span class="cite-bracket">]</span></a></sup> </p><p>Monochorionic twins share the same <a href="/wiki/Placenta" title="Placenta">placenta</a>, and thus have a risk of <a href="/wiki/Twin-to-twin_transfusion_syndrome" title="Twin-to-twin transfusion syndrome">twin-to-twin transfusion syndrome</a>. </p> </td> <td>Days 4–8 </td></tr> <tr> <td><a href="/wiki/Monoamniotic_twins" title="Monoamniotic twins">Monochorionic–Monoamniotic</a> </td> <td>Monochorionic twins share the same <a href="/wiki/Amnion" title="Amnion">amnion</a> in 1–2% of monozygotic twin pregnancies.<sup id="cite_ref-splitstats_47-2" class="reference"><a href="#cite_note-splitstats-47"><span class="cite-bracket">[</span>47<span class="cite-bracket">]</span></a></sup> <p>Monoamniotic twins are <i>always</i> <a href="/wiki/Monozygotic" class="mw-redirect" title="Monozygotic">monozygotic</a>.<sup id="cite_ref-pregnancy-info_50-0" class="reference"><a href="#cite_note-pregnancy-info-50"><span class="cite-bracket">[</span>50<span class="cite-bracket">]</span></a></sup> </p><p>The survival rate for monoamniotic twins is somewhere between 50%<sup id="cite_ref-pregnancy-info_50-1" class="reference"><a href="#cite_note-pregnancy-info-50"><span class="cite-bracket">[</span>50<span class="cite-bracket">]</span></a></sup> and 60%.<sup id="cite_ref-about_51-0" class="reference"><a href="#cite_note-about-51"><span class="cite-bracket">[</span>51<span class="cite-bracket">]</span></a></sup> </p><p>Monoamniotic twins, as with diamniotic monochorionic twins, have a risk of <a href="/wiki/Twin-to-twin_transfusion_syndrome" title="Twin-to-twin transfusion syndrome">twin-to-twin transfusion syndrome</a>. Also, the two umbilical cords have an increased chance of being tangled around the babies. Because of this, there is an increased chance that the newborns may be miscarried or suffer from <a href="/wiki/Cerebral_palsy" title="Cerebral palsy">cerebral palsy</a> due to lack of oxygen. </p> </td> <td>Monoamniotic twins occur when the split takes place after the ninth day after fertilization.<sup id="cite_ref-Shulman_46-4" class="reference"><a href="#cite_note-Shulman-46"><span class="cite-bracket">[</span>46<span class="cite-bracket">]</span></a></sup> </td></tr> <tr> <td><a href="/wiki/Conjoined_twins" title="Conjoined twins">Conjoined twins</a> </td> <td> <p>When the division of the developing <a href="/wiki/Zygote" title="Zygote">zygote</a> into 2 embryos occurs, 99% of the time it is within 8 days of fertilization. </p><p>Mortality is highest for conjoined twins due to the many complications resulting from shared organs. </p> </td> <td>If the division of the zygote occurs later than the 12 days then <a href="/wiki/Conjoined_twins" title="Conjoined twins">conjoined twins</a> are usually the result. </td></tr></tbody></table> <ul class="gallery mw-gallery-packed"> <li class="gallerybox" style="width: 258px"> <div class="thumb" style="width: 256px;"><span typeof="mw:File"><a href="/wiki/File:Dichorionic_twins_with_heart_activity_at_8w5d_since_co-incubation.gif" class="mw-file-description" title="Dichorionic-diamniotic twins at 8 weeks and 5 days since co-incubation as part of IVF. The twin at left in the image is shown in the sagittal plane with the head pointing towards upper left. The twin at right in the image is shown in the coronal plane with the head pointing rightwards."><img alt="Dichorionic-diamniotic twins at 8 weeks and 5 days since co-incubation as part of IVF. The twin at left in the image is shown in the sagittal plane with the head pointing towards upper left. The twin at right in the image is shown in the coronal plane with the head pointing rightwards." src="//upload.wikimedia.org/wikipedia/commons/thumb/a/a9/Dichorionic_twins_with_heart_activity_at_8w5d_since_co-incubation.gif/384px-Dichorionic_twins_with_heart_activity_at_8w5d_since_co-incubation.gif" decoding="async" width="256" height="180" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/a/a9/Dichorionic_twins_with_heart_activity_at_8w5d_since_co-incubation.gif 1.5x" data-file-width="401" data-file-height="282" /></a></span></div> <div class="gallerytext">Dichorionic-diamniotic twins at 8 weeks and 5 days since <a href="/wiki/Co-incubation" class="mw-redirect" title="Co-incubation">co-incubation</a> as part of IVF. The twin at left in the image is shown in the <a href="/wiki/Sagittal_plane" title="Sagittal plane">sagittal plane</a> with the head pointing towards upper left. The twin at right in the image is shown in the <a href="/wiki/Coronal_plane" title="Coronal plane">coronal plane</a> with the head pointing rightwards.</div> </li> <li class="gallerybox" style="width: 229.33333333333px"> <div class="thumb" style="width: 227.33333333333px;"><span typeof="mw:File"><a href="/wiki/File:Monoamniotic_twins_at_15_weeks.jpg" class="mw-file-description" title="Abdominal ultrasonography of monoamniotic twins at a gestational age of 15 weeks. There is no sign of any membrane between the fetuses. A coronal plane is shown of the twin at left, and a sagittal plane of parts of the upper thorax and head is shown of the twin at right."><img alt="Abdominal ultrasonography of monoamniotic twins at a gestational age of 15 weeks. There is no sign of any membrane between the fetuses. A coronal plane is shown of the twin at left, and a sagittal plane of parts of the upper thorax and head is shown of the twin at right." src="//upload.wikimedia.org/wikipedia/commons/thumb/7/73/Monoamniotic_twins_at_15_weeks.jpg/341px-Monoamniotic_twins_at_15_weeks.jpg" decoding="async" width="228" height="180" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/7/73/Monoamniotic_twins_at_15_weeks.jpg/511px-Monoamniotic_twins_at_15_weeks.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/7/73/Monoamniotic_twins_at_15_weeks.jpg 2x" data-file-width="545" data-file-height="432" /></a></span></div> <div class="gallerytext"><a href="/wiki/Abdominal_ultrasonography" title="Abdominal ultrasonography">Abdominal ultrasonography</a> of monoamniotic twins at a <a href="/wiki/Gestational_age_(obstetrics)" class="mw-redirect" title="Gestational age (obstetrics)">gestational age</a> of 15 weeks. There is no sign of any membrane between the fetuses. A <a href="/wiki/Coronal_plane" title="Coronal plane">coronal plane</a> is shown of the twin at left, and a <a href="/wiki/Sagittal_plane" title="Sagittal plane">sagittal plane</a> of parts of the upper thorax and head is shown of the twin at right.</div> </li> </ul> <div class="mw-heading mw-heading2"><h2 id="Demographics">Demographics</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Twin&action=edit&section=10" title="Edit section: Demographics"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>A 2006 study has found that <a href="/wiki/Insulin-like_growth_factor" title="Insulin-like growth factor">insulin-like growth factor</a> present in dairy products may increase the chance of dizygotic twinning. Specifically, the study found that <a href="/wiki/Veganism" title="Veganism">vegan</a> mothers (who exclude dairy from their diets) are one-fifth as likely to have twins as vegetarian or omnivore mothers, and concluded that "Genotypes favoring elevated IGF and diets including dairy products, especially in areas where <a href="/wiki/Growth_hormone" title="Growth hormone">growth hormone</a> is given to cattle, appear to enhance the chances of multiple pregnancies due to ovarian stimulation."<sup id="cite_ref-vegan_52-0" class="reference"><a href="#cite_note-vegan-52"><span class="cite-bracket">[</span>52<span class="cite-bracket">]</span></a></sup> </p><p>From 1980 to 1997, the number of twin births in the United States rose 52%.<sup id="cite_ref-53" class="reference"><a href="#cite_note-53"><span class="cite-bracket">[</span>53<span class="cite-bracket">]</span></a></sup> This rise can at least partly be attributed to the increasing popularity of <a href="/wiki/Fertility_medication" title="Fertility medication">fertility drugs</a> and procedures such as IVF, which result in multiple births more frequently than unassisted fertilizations do. It may also be linked to the increase of <a href="/wiki/Growth_hormone" title="Growth hormone">growth hormones</a> in food.<sup id="cite_ref-vegan_52-1" class="reference"><a href="#cite_note-vegan-52"><span class="cite-bracket">[</span>52<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading3"><h3 id="Ethnicity">Ethnicity</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Twin&action=edit&section=11" title="Edit section: Ethnicity"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1236090951"><div role="note" class="hatnote navigation-not-searchable">Main article: <a href="/wiki/Populated_places_with_highest_incidence_of_multiple_birth" title="Populated places with highest incidence of multiple birth">Populated places with highest incidence of multiple birth</a></div> <figure class="mw-default-size mw-halign-right" typeof="mw:File/Thumb"><a href="/wiki/File:The_Childrens_Museum_of_Indianapolis_-_Female_ere_ibeji_twin_figure_pair.jpg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/e/e1/The_Childrens_Museum_of_Indianapolis_-_Female_ere_ibeji_twin_figure_pair.jpg/170px-The_Childrens_Museum_of_Indianapolis_-_Female_ere_ibeji_twin_figure_pair.jpg" decoding="async" width="170" height="227" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/e/e1/The_Childrens_Museum_of_Indianapolis_-_Female_ere_ibeji_twin_figure_pair.jpg/255px-The_Childrens_Museum_of_Indianapolis_-_Female_ere_ibeji_twin_figure_pair.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/e/e1/The_Childrens_Museum_of_Indianapolis_-_Female_ere_ibeji_twin_figure_pair.jpg/340px-The_Childrens_Museum_of_Indianapolis_-_Female_ere_ibeji_twin_figure_pair.jpg 2x" data-file-width="480" data-file-height="640" /></a><figcaption>A pair of female <a href="/wiki/Ibeji" title="Ibeji">ere ibeji</a> twin figures (early 20th-century) in the permanent collection of <a href="/wiki/The_Children%27s_Museum_of_Indianapolis" title="The Children's Museum of Indianapolis">The Children's Museum of Indianapolis</a>. The <a href="/wiki/Yoruba_people" title="Yoruba people">Yoruba people</a> have the highest dizygotic twinning rate in the world.</figcaption></figure> <p>About 1 in 90 human births (1.1%) results from a twin pregnancy.<sup id="cite_ref-asch_54-0" class="reference"><a href="#cite_note-asch-54"><span class="cite-bracket">[</span>54<span class="cite-bracket">]</span></a></sup> The rate of <i>dizygotic</i> twinning varies greatly among <a href="/wiki/Ethnicity" title="Ethnicity">ethnic groups</a>, ranging as high as about 45 per 1000 births (4.5%) for the <a href="/wiki/Yoruba_people" title="Yoruba people">Yoruba</a> to 10% for Linha São Pedro, a tiny Brazilian settlement which belongs to the city of <a href="/wiki/C%C3%A2ndido_God%C3%B3i" title="Cândido Godói">Cândido Godói</a>.<sup id="cite_ref-55" class="reference"><a href="#cite_note-55"><span class="cite-bracket">[</span>55<span class="cite-bracket">]</span></a></sup> In Cândido Godói, one in five pregnancies has resulted in twins.<sup id="cite_ref-56" class="reference"><a href="#cite_note-56"><span class="cite-bracket">[</span>56<span class="cite-bracket">]</span></a></sup> The Argentine historian Jorge Camarasa has put forward the theory that experiments of the <a href="/wiki/Nazi" class="mw-redirect" title="Nazi">Nazi</a> doctor <a href="/wiki/Josef_Mengele" title="Josef Mengele">Josef Mengele</a> could be responsible for the high ratio of twins in the area. His theory was rejected by Brazilian scientists who had studied twins living in Linha São Pedro; they suggested genetic factors within that community as a more likely explanation.<sup id="cite_ref-57" class="reference"><a href="#cite_note-57"><span class="cite-bracket">[</span>57<span class="cite-bracket">]</span></a></sup> A high twinning rate has also been observed in other places of the world, including: </p> <ul><li><a href="/wiki/Igbo-Ora" title="Igbo-Ora">Igbo-Ora</a> in <a href="/wiki/Nigeria" title="Nigeria">Nigeria</a><sup id="cite_ref-58" class="reference"><a href="#cite_note-58"><span class="cite-bracket">[</span>58<span class="cite-bracket">]</span></a></sup></li> <li><a href="/wiki/Kodinhi" title="Kodinhi">Kodinhi</a>, located in <a href="/wiki/Kerala" title="Kerala">Kerala</a>, India<sup id="cite_ref-59" class="reference"><a href="#cite_note-59"><span class="cite-bracket">[</span>59<span class="cite-bracket">]</span></a></sup></li> <li><a href="/wiki/Mohammadpur_Umri" title="Mohammadpur Umri">Mohammadpur Umri</a>, located in <a href="/wiki/Uttar_Pradesh" title="Uttar Pradesh">Uttar Pradesh</a>, India<sup id="cite_ref-60" class="reference"><a href="#cite_note-60"><span class="cite-bracket">[</span>60<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-61" class="reference"><a href="#cite_note-61"><span class="cite-bracket">[</span>61<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-62" class="reference"><a href="#cite_note-62"><span class="cite-bracket">[</span>62<span class="cite-bracket">]</span></a></sup></li></ul> <p>The widespread use of fertility drugs causing hyperovulation (stimulated release of multiple eggs by the mother) has caused what some call an "epidemic of <a href="/wiki/Multiple_birth" title="Multiple birth">multiple births</a>". In 2001, for the first time ever in the <a href="/wiki/United_States" title="United States">US</a>, the twinning rate exceeded 3% of all births. Nevertheless, the rate of <i>monozygotic</i> twins remains at about 1 in 333 across the globe.<sup class="noprint Inline-Template Template-Fact" style="white-space:nowrap;">[<i><a href="/wiki/Wikipedia:Citation_needed" title="Wikipedia:Citation needed"><span title="This claim needs references to reliable sources. (February 2024)">citation needed</span></a></i>]</sup> </p><p>In a study on the maternity records of 5750 <a href="/wiki/Hausa_people" title="Hausa people">Hausa</a> women living in the Savannah zone of <a href="/wiki/Nigeria" title="Nigeria">Nigeria</a>, there were 40 twins and 2 triplets per 1000 births. Twenty-six percent of twins were monozygotic. The incidence of multiple births, which was about five times higher than that observed in any western population, was significantly lower than that of other ethnic groups, who live in the hot and humid climate of the southern part of the country. The incidence of multiple births was related to maternal age but did not bear any association to the climate or prevalence of <a href="/wiki/Malaria" title="Malaria">malaria</a>.<sup id="cite_ref-hausa_63-0" class="reference"><a href="#cite_note-hausa-63"><span class="cite-bracket">[</span>63<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-64" class="reference"><a href="#cite_note-64"><span class="cite-bracket">[</span>64<span class="cite-bracket">]</span></a></sup> </p><p>Twins are more common in people of African descent.<sup id="cite_ref-65" class="reference"><a href="#cite_note-65"><span class="cite-bracket">[</span>65<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading3"><h3 id="Predisposing_factors">Predisposing factors</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Twin&action=edit&section=12" title="Edit section: Predisposing factors"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>The predisposing factors of monozygotic twinning are unknown. </p><p>Dizygotic twin pregnancies are slightly more likely when the following factors are present in the woman: </p> <ul><li>She is of <a href="/wiki/West_Africa" title="West Africa">West African</a> descent (especially <a href="/wiki/Yoruba_people" title="Yoruba people">Yoruba</a>)</li> <li>She is between the age of 30 and 40 years</li> <li>She is greater than average height and weight</li> <li>She has had several previous pregnancies.</li></ul> <p>Women undergoing certain fertility treatments may have a greater chance of dizygotic multiple births. In the United States it has been estimated that by 2011 36% of twin births resulted from conception by <a href="/wiki/Assisted_reproductive_technology" title="Assisted reproductive technology">assisted reproductive technology</a>.<sup id="cite_ref-66" class="reference"><a href="#cite_note-66"><span class="cite-bracket">[</span>66<span class="cite-bracket">]</span></a></sup> </p><p>The risk of twin birth can vary depending on what types of fertility treatments are used. With <a href="/wiki/In_vitro_fertilisation" title="In vitro fertilisation">in vitro fertilisation</a> (IVF), this is primarily due to the insertion of multiple embryos into the uterus. <a href="/wiki/Controlled_ovarian_hyperstimulation" title="Controlled ovarian hyperstimulation">Ovarian hyperstimulation</a> without IVF has a very high risk of multiple birth. Reversal of <a href="/wiki/Anovulation" title="Anovulation">anovulation</a> with <a href="/wiki/Clomifene" title="Clomifene">clomifene</a> (trade names including <i>Clomid</i>) has a relatively less but yet significant risk of multiple pregnancy. </p> <div class="mw-heading mw-heading2"><h2 id="Delivery_interval">Delivery interval</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Twin&action=edit&section=13" title="Edit section: Delivery interval"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>A 15-year German study<sup id="cite_ref-acta_67-0" class="reference"><a href="#cite_note-acta-67"><span class="cite-bracket">[</span>67<span class="cite-bracket">]</span></a></sup> of 8,220 vaginally delivered twins (that is, 4,110 pregnancies) in <a href="/wiki/Hesse" title="Hesse">Hesse</a> yielded a mean delivery time interval of 13.5 minutes.<sup id="cite_ref-68" class="reference"><a href="#cite_note-68"><span class="cite-bracket">[</span>68<span class="cite-bracket">]</span></a></sup> The delivery interval between the twins was measured as follows: </p> <ul><li>Within 15 minutes: 75.8%</li> <li>16–30 minutes: 16.4%</li> <li>31–45 minutes: 4.3%</li> <li>46–60 minutes: 1.7%</li> <li>Over 60 minutes: 1.8%</li></ul> <p>The study stated that the occurrence of complications "was found to be more likely with increasing twin-to-twin delivery time interval" and suggested that the interval be kept short, though it noted that the study did not examine causes of complications and did not control for factors such as the level of experience of the obstetrician, the wish of the women giving birth, or the "management strategies" of the procedure of delivering the second twin. </p><p>There have also been cases in which twins are born a number of days apart. Possibly the worldwide record for the duration of the time gap between the first and the second delivery was the birth of twins 97 days apart in Cologne, Germany, the first of which was born on November 17, 2018.<sup id="cite_ref-69" class="reference"><a href="#cite_note-69"><span class="cite-bracket">[</span>69<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading2"><h2 id="Complications_during_pregnancy">Complications during pregnancy</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Twin&action=edit&section=14" title="Edit section: Complications during pregnancy"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <div class="mw-heading mw-heading3"><h3 id="Vanishing_twins">Vanishing twins</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Twin&action=edit&section=15" title="Edit section: Vanishing twins"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1236090951"><div role="note" class="hatnote navigation-not-searchable">Main article: <a href="/wiki/Vanishing_twin" title="Vanishing twin">Vanishing twin</a></div> <p>Researchers suspect that as many as 1 in 8 pregnancies start out as multiples, but only a single fetus is brought to full term, because the other fetus has died very early in the pregnancy and has not been detected or recorded.<sup id="cite_ref-Keith-2_70-0" class="reference"><a href="#cite_note-Keith-2-70"><span class="cite-bracket">[</span>70<span class="cite-bracket">]</span></a></sup> Early <a href="/wiki/Obstetric_ultrasonography" title="Obstetric ultrasonography">obstetric ultrasonography</a> exams sometimes reveal an "extra" fetus, which fails to develop and instead disintegrates and vanishes in the uterus. There are several reasons for the "vanishing" fetus, including it being embodied or absorbed by the other fetus, placenta or the mother. This is known as <a href="/wiki/Vanishing_twin" title="Vanishing twin">vanishing twin</a> syndrome. Also, in an unknown proportion of cases, two zygotes may fuse soon after fertilization, resulting in a single <a href="/wiki/Chimera_(genetics)" title="Chimera (genetics)">chimeric</a> embryo, and, later, fetus. </p> <div class="mw-heading mw-heading3"><h3 id="Conjoined_twins">Conjoined twins</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Twin&action=edit&section=16" title="Edit section: Conjoined twins"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1236090951"><div role="note" class="hatnote navigation-not-searchable">Main article: <a href="/wiki/Conjoined_twins" title="Conjoined twins">Conjoined twins</a></div> <figure class="mw-default-size" typeof="mw:File/Thumb"><a href="/wiki/File:Chang-eng-bunker-PD.png" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/a/a8/Chang-eng-bunker-PD.png/170px-Chang-eng-bunker-PD.png" decoding="async" width="170" height="235" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/a/a8/Chang-eng-bunker-PD.png/255px-Chang-eng-bunker-PD.png 1.5x, //upload.wikimedia.org/wikipedia/commons/a/a8/Chang-eng-bunker-PD.png 2x" data-file-width="333" data-file-height="461" /></a><figcaption><a href="/wiki/Chang_and_Eng_Bunker" title="Chang and Eng Bunker">Chang and Eng Bunker</a>, born in Siam (now <a href="/wiki/Thailand" title="Thailand">Thailand</a>) in 1811, were the origin of the term "Siamese twins".</figcaption></figure> <p>Conjoined twins (or the once-commonly used term "siamese") are monozygotic twins whose bodies are joined during pregnancy. This occurs when the zygote starts to split after day 12<sup id="cite_ref-Shulman_46-5" class="reference"><a href="#cite_note-Shulman-46"><span class="cite-bracket">[</span>46<span class="cite-bracket">]</span></a></sup> following fertilization and fails to separate completely. This condition occurs in about 1 in 50,000 human pregnancies. Most conjoined twins are now evaluated for surgery to attempt to separate them into separate functional bodies. The degree of difficulty rises if a vital organ or structure is shared between twins, such as the <a href="/wiki/Brain" title="Brain">brain</a>, <a href="/wiki/Heart" title="Heart">heart</a>, <a href="/wiki/Liver" title="Liver">liver</a> or <a href="/wiki/Lung" title="Lung">lungs</a>. </p> <div class="mw-heading mw-heading3"><h3 id="Chimerism">Chimerism</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Twin&action=edit&section=17" title="Edit section: Chimerism"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1236090951"><div role="note" class="hatnote navigation-not-searchable">Main article: <a href="/wiki/Chimera_(genetics)" title="Chimera (genetics)">Chimera (genetics)</a></div> <p>A chimera is an ordinary person or animal except that some of their parts actually came from their twin or from the mother. A chimera may arise either from monozygotic twin fetuses (where it would be impossible to detect), or from dizygotic fetuses, which can be identified by chromosomal comparisons from various parts of the body. The number of cells derived from each fetus can vary from one part of the body to another, and often leads to characteristic <a href="/wiki/Mosaicism" class="mw-redirect" title="Mosaicism">mosaicism</a> skin coloration in human chimeras. A chimera may be <a href="/wiki/Intersex" title="Intersex">intersex</a>, composed of cells from a male twin and a female twin. In one case <a href="/wiki/DNA_test" class="mw-redirect" title="DNA test">DNA tests</a> determined that a woman, <a href="/wiki/Lydia_Fairchild" title="Lydia Fairchild">Lydia Fairchild</a>, mystifyingly, was not the mother of two of her three children; she was found to be a chimera, and the two children were conceived from eggs derived from cells of their mother's twin.<sup id="cite_ref-71" class="reference"><a href="#cite_note-71"><span class="cite-bracket">[</span>71<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading3"><h3 id="Parasitic_twins">Parasitic twins</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Twin&action=edit&section=18" title="Edit section: Parasitic twins"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1236090951"><div role="note" class="hatnote navigation-not-searchable">Main article: <a href="/wiki/Parasitic_twin" title="Parasitic twin">Parasitic twin</a></div> <p>Sometimes one twin fetus will fail to develop completely and continue to cause problems for its surviving twin. One fetus acts as a <a href="/wiki/Parasite" class="mw-redirect" title="Parasite">parasite</a> towards the other. Sometimes the parasitic twin becomes an almost indistinguishable part of the other, and sometimes this needs to be treated medically. </p> <div class="mw-heading mw-heading3"><h3 id="Partial_molar_twins">Partial molar twins</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Twin&action=edit&section=19" title="Edit section: Partial molar twins"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>A very rare type of parasitic twinning is one where a single viable twin is endangered when the other zygote becomes cancerous, or "molar". This means that the molar zygote's cellular division continues unchecked, resulting in a cancerous growth that overtakes the viable fetus. Typically, this results when one twin has either <a href="/wiki/Triploidy" class="mw-redirect" title="Triploidy">triploidy</a> or complete paternal <a href="/wiki/Uniparental_disomy" title="Uniparental disomy">uniparental disomy</a>, resulting in little or no fetus and a cancerous, overgrown placenta, resembling a <a href="/wiki/Bunch_of_grapes" class="mw-redirect" title="Bunch of grapes">bunch of grapes</a>. </p> <div class="mw-heading mw-heading3"><h3 id="Miscarried_twin">Miscarried twin</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Twin&action=edit&section=20" title="Edit section: Miscarried twin"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Occasionally, a woman will suffer a <a href="/wiki/Miscarriage" title="Miscarriage">miscarriage</a> early in pregnancy, yet the pregnancy will continue; one twin was miscarried but the other was able to be carried to term. This occurrence is similar to the <a href="/wiki/Vanishing_twin" title="Vanishing twin">vanishing twin</a> syndrome, but typically occurs later, as the twin is not reabsorbed. </p> <div class="mw-heading mw-heading3"><h3 id="Low_birth_weight">Low birth weight</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Twin&action=edit&section=21" title="Edit section: Low birth weight"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>It is very common for twins to be born at a low <a href="/wiki/Birth_weight" title="Birth weight">birth weight</a>. More than half of twins are born weighing less than 5.5 pounds (2.5 kg), while the average birth weight of a healthy baby should be around 6–8 pounds (3–4 kg).<sup id="cite_ref-72" class="reference"><a href="#cite_note-72"><span class="cite-bracket">[</span>72<span class="cite-bracket">]</span></a></sup> This is largely due to the fact that twins are typically born <a href="/wiki/Premature_labour" class="mw-redirect" title="Premature labour">premature</a>. Premature birth and low birth weights, especially when under 3.5 pounds (1.6 kg), can increase the risk of several health-related issues, such as vision and hearing loss, mental disabilities, and <a href="/wiki/Cerebral_palsy" title="Cerebral palsy">cerebral palsy</a>.<sup id="cite_ref-73" class="reference"><a href="#cite_note-73"><span class="cite-bracket">[</span>73<span class="cite-bracket">]</span></a></sup> There is an increased possibility of potential complications as the birth weight of the baby decreases. </p> <div class="mw-heading mw-heading3"><h3 id="Twin-to-twin_transfusion_syndrome">Twin-to-twin transfusion syndrome</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Twin&action=edit&section=22" title="Edit section: Twin-to-twin transfusion syndrome"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1236090951"><div role="note" class="hatnote navigation-not-searchable">Main article: <a href="/wiki/Twin-to-twin_transfusion_syndrome" title="Twin-to-twin transfusion syndrome">Twin-to-twin transfusion syndrome</a></div> <figure class="mw-default-size" typeof="mw:File/Thumb"><a href="/wiki/File:Tweeling_transfusiesyndroom.jpg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/c/ca/Tweeling_transfusiesyndroom.jpg/220px-Tweeling_transfusiesyndroom.jpg" decoding="async" width="220" height="185" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/c/ca/Tweeling_transfusiesyndroom.jpg/330px-Tweeling_transfusiesyndroom.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/c/ca/Tweeling_transfusiesyndroom.jpg/440px-Tweeling_transfusiesyndroom.jpg 2x" data-file-width="2885" data-file-height="2420" /></a><figcaption>Twin-to-twin transfusion syndrome (TTTS) illustration of twins showing one fetus with exposure to more amniotic fluid while the other is "stuck" with the membrane tightly around itself</figcaption></figure> <p>Monozygotic twins who share a placenta can develop twin-to-twin transfusion syndrome. This condition means that blood from one twin is being diverted into the other twin. One twin, the 'donor' twin, is small and <a href="/wiki/Anemic" class="mw-redirect" title="Anemic">anemic</a>, the other, the 'recipient' twin, is large and <a href="/wiki/Polycythemic" class="mw-redirect" title="Polycythemic">polycythemic</a>. The lives of both twins are endangered by this condition. </p> <div class="mw-heading mw-heading3"><h3 id="Stillbirths">Stillbirths</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Twin&action=edit&section=23" title="Edit section: Stillbirths"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p><a href="/wiki/Stillbirth" title="Stillbirth">Stillbirths</a> occurs when a fetus dies after 20 weeks of gestation. There are two types of stillbirth, including intrauterine death and intrapartum death. Intrauterine death occurs when a baby dies during late pregnancy. Intrapartum death, which is more common, occurs when a baby dies while the mother is giving birth. The cause of stillbirth is often unknown, but the rate of babies who are stillborn is higher in twins and multiple births. Caesareans or inductions are advised after 38 weeks of pregnancy for twins, because the risk of stillbirth increases after this time.<sup id="cite_ref-74" class="reference"><a href="#cite_note-74"><span class="cite-bracket">[</span>74<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading3"><h3 id="Heterotopic_pregnancy">Heterotopic pregnancy</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Twin&action=edit&section=24" title="Edit section: Heterotopic pregnancy"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p><a href="/wiki/Heterotopic_pregnancy" title="Heterotopic pregnancy">Heterotopic pregnancy</a> is an exceedingly rare type of dizygotic twinning in which one twin implants in the uterus as normal and the other remains in the fallopian tube as an <a href="/wiki/Ectopic_pregnancy" title="Ectopic pregnancy">ectopic pregnancy</a>. Ectopic pregnancies must be resolved because they can be life-threatening to the mother. However, in most cases, the intrauterine pregnancy can be salvaged.<sup class="noprint Inline-Template Template-Fact" style="white-space:nowrap;">[<i><a href="/wiki/Wikipedia:Citation_needed" title="Wikipedia:Citation needed"><span title="This claim needs references to reliable sources. (September 2015)">citation needed</span></a></i>]</sup> </p> <div class="mw-heading mw-heading2"><h2 id="Management_of_birth">Management of birth</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Twin&action=edit&section=25" title="Edit section: Management of birth"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>For otherwise healthy twin pregnancies where both twins are head down a trial of <a href="/wiki/Vaginal_delivery" title="Vaginal delivery">vaginal delivery</a> is recommended at between 37 and 38 weeks.<sup id="cite_ref-Bis2013_75-0" class="reference"><a href="#cite_note-Bis2013-75"><span class="cite-bracket">[</span>75<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-NICE2011Chp5_76-0" class="reference"><a href="#cite_note-NICE2011Chp5-76"><span class="cite-bracket">[</span>76<span class="cite-bracket">]</span></a></sup> Vaginal delivery in this case does not worsen the outcome for the infant as compared with <a href="/wiki/Caesarean_section" title="Caesarean section">Caesarean section</a>.<sup id="cite_ref-Bis2013_75-1" class="reference"><a href="#cite_note-Bis2013-75"><span class="cite-bracket">[</span>75<span class="cite-bracket">]</span></a></sup> There is controversy on the best method of delivery where the first twin is head first and the second is not.<sup id="cite_ref-Bis2013_75-2" class="reference"><a href="#cite_note-Bis2013-75"><span class="cite-bracket">[</span>75<span class="cite-bracket">]</span></a></sup> When the first twin is not head down a caesarean section is often recommended.<sup id="cite_ref-Bis2013_75-3" class="reference"><a href="#cite_note-Bis2013-75"><span class="cite-bracket">[</span>75<span class="cite-bracket">]</span></a></sup> It is estimated that 75% of twin pregnancies in the United States were delivered by caesarean section in 2008.<sup id="cite_ref-pmid22015878_77-0" class="reference"><a href="#cite_note-pmid22015878-77"><span class="cite-bracket">[</span>77<span class="cite-bracket">]</span></a></sup> In comparison, the rate of caesarean section for all pregnancies in the general population varies between 14% and 40%.<sup id="cite_ref-78" class="reference"><a href="#cite_note-78"><span class="cite-bracket">[</span>78<span class="cite-bracket">]</span></a></sup> In twins that share the same placenta, delivery may be considered at 36 weeks.<sup id="cite_ref-pmid27599496_79-0" class="reference"><a href="#cite_note-pmid27599496-79"><span class="cite-bracket">[</span>79<span class="cite-bracket">]</span></a></sup> For twins who are born early, there is insufficient evidence for or against placing preterm stable twins in the same cot or incubator (co-bedding).<sup id="cite_ref-80" class="reference"><a href="#cite_note-80"><span class="cite-bracket">[</span>80<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading2"><h2 id="Human_twin_studies">Human twin studies</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Twin&action=edit&section=26" title="Edit section: Human twin studies"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1236090951"><div role="note" class="hatnote navigation-not-searchable">Main article: <a href="/wiki/Twin_study" title="Twin study">Twin study</a></div> <p>Twin studies are utilized in an attempt to determine how much of a particular trait is attributable to either genetics or environmental influence. These studies compare monozygotic and dizygotic twins for <a href="/wiki/Medical" class="mw-redirect" title="Medical">medical</a>, <a href="/wiki/Genetics" title="Genetics">genetic</a>, or <a href="/wiki/Psychology" title="Psychology">psychological</a> characteristics to try to isolate genetic influence from epigenetic and <a href="/wiki/Natural_environment" title="Natural environment">environmental</a> influence. Twins that have been separated early in life and raised in separate households are especially sought-after for these studies, which have been used widely in the exploration of <a href="/wiki/Human_nature" title="Human nature">human nature</a>. Classical twin studies are now being supplemented with molecular genetic studies which identify individual genes. </p> <div class="mw-heading mw-heading2"><h2 id="Unusual_twinnings">Unusual twinnings</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Twin&action=edit&section=27" title="Edit section: Unusual twinnings"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <div class="mw-heading mw-heading3"><h3 id="Bi-paternal_twins">Bi-paternal twins</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Twin&action=edit&section=28" title="Edit section: Bi-paternal twins"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>This phenomenon is known as heteropaternal <a href="/wiki/Superfecundation" title="Superfecundation">superfecundation</a>. One 1992 study estimates that the frequency of heteropaternal superfecundation among dizygotic twins, whose parents were involved in paternity suits, was approximately 2.4%.<sup class="noprint Inline-Template Template-Fact" style="white-space:nowrap;">[<i><a href="/wiki/Wikipedia:Citation_needed" title="Wikipedia:Citation needed"><span title="This claim needs references to reliable sources. (June 2019)">citation needed</span></a></i>]</sup> </p> <div class="mw-heading mw-heading3"><h3 id="Mixed_twins">Mixed twins</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Twin&action=edit&section=29" title="Edit section: Mixed twins"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1236090951"><div role="note" class="hatnote navigation-not-searchable">Main article: <a href="/wiki/Mixed_twins" title="Mixed twins">Mixed twins</a></div> <p>Dizygotic twins from <a href="/wiki/Multiracial_people" title="Multiracial people">biracial</a> couples can sometimes be <a href="/wiki/Mixed_twins" title="Mixed twins">mixed twins</a>, which exhibit differing ethnic and racial features. One such pairing was born in <a href="/wiki/London" title="London">London</a> in 1993 to a white mother and Caribbean father.<sup id="cite_ref-81" class="reference"><a href="#cite_note-81"><span class="cite-bracket">[</span>81<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading3"><h3 id="Monozygotic_twins_of_different_sexes">Monozygotic twins of different sexes</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Twin&action=edit&section=30" title="Edit section: Monozygotic twins of different sexes"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Among monozygotic twins, in extremely rare cases, twins have been born with different sexes (one male, one female).<sup id="cite_ref-82" class="reference"><a href="#cite_note-82"><span class="cite-bracket">[</span>82<span class="cite-bracket">]</span></a></sup> When monozygotic twins are born with different sexes it is because of chromosomal defects. The probability of this is so small that multiples having different sexes is universally accepted as a sound basis for <i>in utero</i> clinical determination that the multiples are not monozygotic. </p><p>Another abnormality that can result in monozygotic twins of different sexes is if the egg is fertilized by a male sperm but during cell division only the X chromosome is duplicated. This results in one normal male (XY) and one female with <a href="/wiki/Turner_syndrome" title="Turner syndrome">Turner syndrome</a> (45,X).<sup id="cite_ref-83" class="reference"><a href="#cite_note-83"><span class="cite-bracket">[</span>83<span class="cite-bracket">]</span></a></sup> In these cases, although the twins did form from the same fertilized egg, it is incorrect to refer to them as genetically identical, since they have different <a href="/wiki/Karyotype" title="Karyotype">karyotypes</a>. </p> <div class="mw-heading mw-heading3"><h3 id="Semi-identical_(sesquizygotic)_twins"><span id="Semi-identical_.28sesquizygotic.29_twins"></span><span class="anchor" id="Dizygotic_(fraternal)_twins"></span><span class="anchor" id="Half-identical_twins"></span>Semi-identical (sesquizygotic) twins</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Twin&action=edit&section=31" title="Edit section: Semi-identical (sesquizygotic) twins"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p><a href="/wiki/Monozygotic" class="mw-redirect" title="Monozygotic">Monozygotic</a> twins can develop differently, due to their genes being differently activated.<sup id="cite_ref-84" class="reference"><a href="#cite_note-84"><span class="cite-bracket">[</span>84<span class="cite-bracket">]</span></a></sup> More unusual are "semi-identical twins", also known as "sesquizygotic". As of 2019<sup class="plainlinks noexcerpt noprint asof-tag update" style="display:none;"><a class="external text" href="https://en.wikipedia.org/w/index.php?title=Twin&action=edit">[update]</a></sup>, only two cases have been reported.<sup id="cite_ref-NEJM_85-0" class="reference"><a href="#cite_note-NEJM-85"><span class="cite-bracket">[</span>85<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-86" class="reference"><a href="#cite_note-86"><span class="cite-bracket">[</span>86<span class="cite-bracket">]</span></a></sup> These "half-identical twins" are hypothesized to occur when an <a href="/wiki/Ovum" class="mw-redirect" title="Ovum">ovum</a> is fertilized by two <a href="/wiki/Sperm" title="Sperm">sperm</a>. The cell assorts the chromosomes by <a href="/wiki/Heterogonesis" title="Heterogonesis">heterogonesis</a> and the cell divides into two, with each daughter cell now containing the correct number of chromosomes. The cells continue to develop into a <a href="/wiki/Morula" class="mw-redirect" title="Morula">morula</a>. If the morula then undergoes a twinning event, two embryos will be formed, with different paternal genes but identical maternal genes.<sup id="cite_ref-87" class="reference"><a href="#cite_note-87"><span class="cite-bracket">[</span>87<span class="cite-bracket">]</span></a></sup> </p> <figure class="mw-default-size mw-halign-right" typeof="mw:File/Thumb"><a href="/wiki/File:Hereford_twins.jpg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/4/4d/Hereford_twins.jpg/220px-Hereford_twins.jpg" decoding="async" width="220" height="145" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/4/4d/Hereford_twins.jpg/330px-Hereford_twins.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/4/4d/Hereford_twins.jpg/440px-Hereford_twins.jpg 2x" data-file-width="2692" data-file-height="1773" /></a><figcaption>Twin calves of the <a href="/wiki/Hereford_(cattle)" class="mw-redirect" title="Hereford (cattle)">Hereford</a> breed in <a href="/wiki/Miles_City,_Montana" title="Miles City, Montana">Miles City, Montana</a></figcaption></figure> <p>In 2007, a study reported a case of a pair of living twins, which shared an identical set of maternal chromosomes, while each having a distinct set of paternal chromosomes, albeit from the same man, and thus they most likely share half of their father's genetic makeup. The twins were both found to be <a href="/wiki/Chimera_(genetics)" title="Chimera (genetics)">chimeras</a>. One was an <a href="/wiki/Intersex" title="Intersex">intersex</a> XX, and one a <a href="/wiki/Phenotypical" class="mw-redirect" title="Phenotypical">XY</a> male. The exact mechanism of fertilization could not be determined but the study stated that it was unlikely to be a case of polar body twinning.<sup id="cite_ref-88" class="reference"><a href="#cite_note-88"><span class="cite-bracket">[</span>88<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Reuters_89-0" class="reference"><a href="#cite_note-Reuters-89"><span class="cite-bracket">[</span>89<span class="cite-bracket">]</span></a></sup> </p><p>The likely genetic basis of semi-identical twins was reported in 2019 by <a href="/wiki/Michael_T._Gabbett" title="Michael T. Gabbett">Michael Gabbett</a> and <a href="/wiki/Nicholas_Fisk_(academic)" class="mw-redirect" title="Nicholas Fisk (academic)">Nicholas Fisk</a>. In their seminal publication, Gabbett, Fisk and colleagues documented a second case of sesquizygosis and presented molecular evidence of the phenomenon.<sup id="cite_ref-NEJM_85-1" class="reference"><a href="#cite_note-NEJM-85"><span class="cite-bracket">[</span>85<span class="cite-bracket">]</span></a></sup> The reported twins shared 100% of their maternal chromosomes and 78% of their paternal genomic information. The authors presented evidence that two <a href="/wiki/Sperm" title="Sperm">sperm</a> from the same man fertilized an <a href="/wiki/Ovum" class="mw-redirect" title="Ovum">ovum</a> simultaneously. The chromosomes assorted themselves through <a href="/wiki/Heterogonesis" title="Heterogonesis">heterogonesis</a> to form three cell lines. The purely paternal cell line died out due to <a href="/wiki/Genomic_imprinting" title="Genomic imprinting">genomic imprinting</a> lethality, while the other two cell lines, each consisting of the same maternal <a href="/wiki/DNA" title="DNA">DNA</a> but only 50% identical paternal DNA, formed a <a href="/wiki/Morula" class="mw-redirect" title="Morula">morula</a> which subsequently split into twins.<sup id="cite_ref-NEJM_85-2" class="reference"><a href="#cite_note-NEJM-85"><span class="cite-bracket">[</span>85<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-90" class="reference"><a href="#cite_note-90"><span class="cite-bracket">[</span>90<span class="cite-bracket">]</span></a></sup> </p><p><span class="anchor" id="chorionicity"></span><span class="anchor" id="amniosity"></span> </p> <div class="mw-heading mw-heading3"><h3 id="Mirror_image_twins">Mirror image twins</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Twin&action=edit&section=32" title="Edit section: Mirror image twins"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Mirror image twins result when a fertilized egg splits later in the embryonic stage than normal timing, around day 9–12. This type of twinning could exhibit characteristics with reversed asymmetry, such as opposite dominant handedness, dental structure, or even organs (<a href="/wiki/Situs_inversus" title="Situs inversus">situs inversus</a>).<sup id="cite_ref-91" class="reference"><a href="#cite_note-91"><span class="cite-bracket">[</span>91<span class="cite-bracket">]</span></a></sup> If the split occurs later than this time period, the twins risk being conjoined. There is no DNA-based zygosity test that can determine if twins are indeed mirror image.<sup id="cite_ref-92" class="reference"><a href="#cite_note-92"><span class="cite-bracket">[</span>92<span class="cite-bracket">]</span></a></sup> The term "mirror image" is used because the twins, when facing each other, appear as matching reflections.<sup id="cite_ref-93" class="reference"><a href="#cite_note-93"><span class="cite-bracket">[</span>93<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading2"><h2 id="Language_development">Language development</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Twin&action=edit&section=33" title="Edit section: Language development"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>There have been many studies highlighting the <a href="/wiki/Language_development" title="Language development">development of language</a> in twins compared to single-born children. These studies have converged on the notion that there is a greater rate of delay in language development in twins compared to their single-born counterparts.<sup id="cite_ref-94" class="reference"><a href="#cite_note-94"><span class="cite-bracket">[</span>94<span class="cite-bracket">]</span></a></sup> The reasons for this phenomenon are still in question; however, <a href="/wiki/Cryptophasia" title="Cryptophasia">cryptophasia</a> was thought to be the major cause.<sup id="cite_ref-95" class="reference"><a href="#cite_note-95"><span class="cite-bracket">[</span>95<span class="cite-bracket">]</span></a></sup> <a href="/wiki/Idioglossia" title="Idioglossia">Idioglossia</a> is defined as a private language that is usually invented by young children, specifically twins. Another term to describe what some people call "twin talk" is cryptophasia where a language is developed by twins that only they can understand. The increased focused communication between two twins may isolate them from the social environment surrounding them. Idioglossia has been found to be a rare occurrence and the attention of scientists has shifted away from this idea. However, there are researchers and scientists that say cryptophasia or idioglossia is not a rare phenomenon. Current research is looking into the impacts of a richer social environment for these twins to stimulate their development of language.<sup id="cite_ref-96" class="reference"><a href="#cite_note-96"><span class="cite-bracket">[</span>96<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading2"><h2 id="Animals">Animals</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Twin&action=edit&section=34" title="Edit section: Animals"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <style data-mw-deduplicate="TemplateStyles:r1251242444">.mw-parser-output .ambox{border:1px solid #a2a9b1;border-left:10px solid #36c;background-color:#fbfbfb;box-sizing:border-box}.mw-parser-output .ambox+link+.ambox,.mw-parser-output .ambox+link+style+.ambox,.mw-parser-output .ambox+link+link+.ambox,.mw-parser-output .ambox+.mw-empty-elt+link+.ambox,.mw-parser-output .ambox+.mw-empty-elt+link+style+.ambox,.mw-parser-output .ambox+.mw-empty-elt+link+link+.ambox{margin-top:-1px}html body.mediawiki .mw-parser-output .ambox.mbox-small-left{margin:4px 1em 4px 0;overflow:hidden;width:238px;border-collapse:collapse;font-size:88%;line-height:1.25em}.mw-parser-output .ambox-speedy{border-left:10px solid #b32424;background-color:#fee7e6}.mw-parser-output .ambox-delete{border-left:10px solid #b32424}.mw-parser-output .ambox-content{border-left:10px solid #f28500}.mw-parser-output .ambox-style{border-left:10px solid #fc3}.mw-parser-output .ambox-move{border-left:10px solid #9932cc}.mw-parser-output .ambox-protection{border-left:10px solid #a2a9b1}.mw-parser-output .ambox .mbox-text{border:none;padding:0.25em 0.5em;width:100%}.mw-parser-output .ambox .mbox-image{border:none;padding:2px 0 2px 0.5em;text-align:center}.mw-parser-output .ambox .mbox-imageright{border:none;padding:2px 0.5em 2px 0;text-align:center}.mw-parser-output .ambox .mbox-empty-cell{border:none;padding:0;width:1px}.mw-parser-output .ambox .mbox-image-div{width:52px}@media(min-width:720px){.mw-parser-output .ambox{margin:0 10%}}@media print{body.ns-0 .mw-parser-output .ambox{display:none!important}}</style><table class="box-Ref_expand plainlinks metadata ambox ambox-content ambox-Refimprove" role="presentation"><tbody><tr><td class="mbox-image"><div class="mbox-image-div"><span typeof="mw:File"><span><img alt="" src="//upload.wikimedia.org/wikipedia/commons/thumb/a/a4/Text_document_with_red_question_mark.svg/40px-Text_document_with_red_question_mark.svg.png" decoding="async" width="40" height="40" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/a/a4/Text_document_with_red_question_mark.svg/60px-Text_document_with_red_question_mark.svg.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/a/a4/Text_document_with_red_question_mark.svg/80px-Text_document_with_red_question_mark.svg.png 2x" data-file-width="48" data-file-height="48" /></span></span></div></td><td class="mbox-text"><div class="mbox-text-span">This section <b>needs more complete <a href="/wiki/Wikipedia:Citing_sources" title="Wikipedia:Citing sources">citations</a> for <a href="/wiki/Wikipedia:Verifiability" title="Wikipedia:Verifiability">verification</a></b>.<span class="hide-when-compact"> Please help <a href="/wiki/Help:Footnotes" title="Help:Footnotes">add missing citation information</a> so that sources are clearly identifiable.</span> <span class="date-container"><i>(<span class="date">March 2024</span>)</i></span><span class="hide-when-compact"><i> (<small><a href="/wiki/Help:Maintenance_template_removal" title="Help:Maintenance template removal">Learn how and when to remove this message</a></small>)</i></span></div></td></tr></tbody></table> <p>Non-human dizygotic twinning is a common phenomenon in multiple animal species, including cats, dogs, cattle, bats, chimpanzees, and deer. This should not be confused with an animal's ability to produce a <a href="/wiki/Litter_(zoology)" title="Litter (zoology)">litter</a>, because while litters are caused by the release of multiple eggs during an <a href="/wiki/Ovulation" title="Ovulation">ovulation</a> cycle, identical to the ovulation of dizygotic twins, they produce more than two offspring. Species such as sheep, goats, and deer have a higher propensity for dizygotic twinning, meaning that they carry a higher frequency of the <a href="/wiki/Allele" title="Allele">allele</a> responsible for the likelihood of twins, rather than the likelihood of litters (Whitcomb, 2021). Cases of <a href="/wiki/Monozygotic_twinning" class="mw-redirect" title="Monozygotic twinning">monozygotic twinning</a> in the animal kingdom are rare but have been recorded on a number of occasions. In 2016, a C-section of an Irish Wolfhound revealed identical twin puppies sharing a singular placenta. South African scientists, who were called in to study the identical twins wrote that... "To the best of our knowledge, this is the first report of monozygotic twinning in the dog confirmed using <a href="/wiki/DNA_profiling" title="DNA profiling">DNA profiling</a>" (Horton, 2016). Additionally, armadillos have also been known to produce monozygotic twins, sometimes birthing two sets of identical twins during one reproductive cycle. Monozygotic twinning in armadillos functions as an evolutionary adaptation preventing inbreeding. Once an armadillo offspring enters its reproductive stage, the organism is forced to leave the nest in search of its mate, rather than mating with its siblings. Not only does monozygotic twinning dissuade from armadillo siblings inbreeding, but by forcing migration from the nest, this adaptation ensures the increased <a href="/wiki/Genetic_variation" title="Genetic variation">genetic variation</a> and geographical population diffusion of armadillo species. </p><p>Due to the increased parental investment provided for their offspring, larger mammals with longer life spans have slower reproductive cycles and tend to birth only one offspring at a time. This commonly repeated behavior in larger mammals evolved as a fixed, naturally-selected adaptation, resulting in a decreased twinning propensity in species such as giraffes, elephants, and hippopotami. Despite this adaptation, a case of rare monozygotic twinning has been documented in two elephant calves at the Bandipur Tiger Reserve in Karnataka, India. Chief Veterinarian of the Wildlife Trust of India, NVK Ashraf, in response to the twinning event, wrote that "in species that invest longer time in producing a baby, taking care of two twin calves will be difficult. Therefore, the incidence of twinning will be comparatively less."Ashraf's insight not only illuminates the rarity of twinning among large mammals in the natural world, but directs our attention to the increased twinning propensity of animals under human care. This increased twinning propensity is thought to be either caused by random mutation facilitated by genetic drift, or the positive selection of the "twinning" trait in human-controlled conditions. Due to the removal of natural predators and unpredictable environmental conditions with the increase of human-provided food and medical care, species residing in nature reserves, zoos, etc., carry an increased likelihood of reversing their naturally-selected traits that have been passed on for generations. When considering this phenomenon in relation to twinning, larger mammals not commonly associated with high twinning propensities can perhaps produce twins as an adaptive response to their human-controlled environment. Additionally, the high twinning propensity in species is thought to be positively correlated with the infant mortality rate of the reproducing organism's environment (Rickard, 2022, p. 2). Thus if a species lives in a controlled environment with a low infant mortality rate, the frequency of the "twinning trait" could increase, leading to a higher likelihood of producing twin offspring. In the case of the monozygotic twin calves in India, their existence could be connected to a new, positively selected adaptation of twinning attributed to species living under human care (Ward, 2014, p. 7-11). </p><p>Species with small physicalities and quick reproductive cycles carry high twinning propensities as a result of increased predation and high mortality rates. As scientists continue to study the origin of dizygotic twinning in the animal kingdom, many have turned to species that demonstrated an increased output of twins during periods of evolutionary distress and natural selection. Through their studies on <a href="/wiki/Vespertilionidae" title="Vespertilionidae">Vespertilionidae</a> and <a href="/wiki/Cebidae" title="Cebidae">Cebidae</a> species, scientists Guilherme Siniciato Terra Garbino (2021) and Marco Varella (2018) have proven that smaller species experiencing infertility in old age and/or unstable habits as a result of increased predation or human interference can experience have undergone natural selection in gaining even higher twinning propensities. In his study on the evolution of litter size in bats, Garbino discovered that the vespertilionidae genus has higher twinning propensities as a result of their high roosting habitats. When tracked phylogenetically, scientists determined that the common ancestor of bats carried a higher twinning propensity which was then lost, and picked up again, eighteen times in evolutionary history. While other bat subfamilies such as Myotinae and Murinae inevitably lost the twinning trait, the family Vespertilionidae retained a high trait frequency due to mutation and environmental conditions that triggered natural selection. The height and exposed nature of Vespertilionidae's roosting locations resulted in a sharp increase in species mortality rate. Natural selection offsets these dangers by positively selecting high twinning propensity, resulting in not only Vespertilionidae's increased ability to produce twins but the increased likelihood of the genus's reproductive survival. This means that despite the family's high exposure to factors that would seemingly increase mortality rates, Vespertilionidae counteracts their environmental conditions through the evolutionary adaptation of dizygotic twins. </p><p>The prevalence of dizygotic twinning in monkeys is thought to be an "insurance adaptation" for mothers reproducing at the end of their fertile years. While dizygotic twinning has been observed in species such as gorillas and chimpanzees, monkeys in the cebidae genus are found to be more likely to produce twins because of their small size and insect-based diet (Varella, 2018). This is because their small size indicates shorter gestation periods and the rapid maturation of offspring, resulting in a shorter lifespan where organisms are rapidly replaced by newer generations. The smaller size of the cebidae genus also makes these species more susceptible to predators, thus triggering the heightened pace of birth, maturation, reproduction, and death. Meanwhile, cebidae's insectivorous existence can be correlated with this genus's heightened ability to reproduce, as more resources become available, more organisms can take advantage of these resources. Thus, monkeys that are smaller and have more access to food, such as the cebidae genus, have the ability to produce more offspring at a quicker pace. In terms of dizygotic twinning, it has been observed that older mothers within the cebidae genus have a higher chance of producing twins than those at the beginning stages of their fertility. Despite their access to resources, the cebidae genus has a high mortality rate attributed to their size, meaning that in order to "keep up" their quickened lifecycle, they must produce an excess of offspring in ensuring generational survival. The positively-selected adaptation of twinning counteracts the genus's high mortality rate by giving older mothers the chance to produce more than one offspring. This not only increases the likelihood that one or more of these offspring will reach reproductive maturity, but gives the mother a chance to birth at least one viable offspring despite their age. Due to their short life cycles, the cebidae genus is more inclined to produce dizygotic twins in their older reproductive years, thus signaling that the trait of high twinning propensity is one that is passed down in service of this genus's survival. </p> <div class="mw-heading mw-heading2"><h2 id="See_also">See also</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Twin&action=edit&section=35" title="Edit section: See also"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <style data-mw-deduplicate="TemplateStyles:r1184024115">.mw-parser-output .div-col{margin-top:0.3em;column-width:30em}.mw-parser-output .div-col-small{font-size:90%}.mw-parser-output .div-col-rules{column-rule:1px solid #aaa}.mw-parser-output .div-col dl,.mw-parser-output .div-col ol,.mw-parser-output .div-col ul{margin-top:0}.mw-parser-output .div-col li,.mw-parser-output .div-col dd{page-break-inside:avoid;break-inside:avoid-column}</style><div class="div-col" style="column-width: 15em;"> <ul><li><a href="/wiki/Cloning" title="Cloning">Cloning</a></li> <li><a href="/wiki/Doppelg%C3%A4nger" title="Doppelgänger">Doppelgänger</a></li> <li><a href="/wiki/Evil_twin" title="Evil twin">Evil twin</a></li> <li><a href="/wiki/Gemini_(astrology)" title="Gemini (astrology)">Gemini (astrology)</a></li> <li><a href="/wiki/List_of_multiple_births" title="List of multiple births">List of multiple births</a></li> <li><a href="/wiki/List_of_twins" title="List of twins">List of notable twins</a></li> <li><a href="/wiki/Litter_(animal)" class="mw-redirect" title="Litter (animal)">Litter (animal)</a></li> <li><a href="/wiki/Look-alike" title="Look-alike">Look-alike</a></li> <li><a href="/wiki/Mixed_twins" title="Mixed twins">Mixed twins</a></li> <li><a href="/wiki/Multiple_birth" title="Multiple birth">Multiple birth</a></li> <li><a href="/wiki/Superfecundation" title="Superfecundation">Superfecundation</a></li> <li><a href="/wiki/Twin_study" title="Twin study">Twin study</a></li> <li><a href="/wiki/Twin_towns_and_sister_cities" class="mw-redirect" title="Twin towns and sister cities">Twin towns and sister cities</a></li> <li><a href="/wiki/Twins_in_mythology" title="Twins in mythology">Twins in mythology</a></li> <li><a href="/wiki/Virtual_twin" title="Virtual twin">Virtual twin</a></li></ul> </div> <div class="mw-heading mw-heading2"><h2 id="References">References</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Twin&action=edit&section=36" title="Edit section: References"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <style data-mw-deduplicate="TemplateStyles:r1239543626">.mw-parser-output .reflist{margin-bottom:0.5em;list-style-type:decimal}@media screen{.mw-parser-output .reflist{font-size:90%}}.mw-parser-output .reflist .references{font-size:100%;margin-bottom:0;list-style-type:inherit}.mw-parser-output .reflist-columns-2{column-width:30em}.mw-parser-output .reflist-columns-3{column-width:25em}.mw-parser-output .reflist-columns{margin-top:0.3em}.mw-parser-output .reflist-columns ol{margin-top:0}.mw-parser-output .reflist-columns li{page-break-inside:avoid;break-inside:avoid-column}.mw-parser-output .reflist-upper-alpha{list-style-type:upper-alpha}.mw-parser-output .reflist-upper-roman{list-style-type:upper-roman}.mw-parser-output .reflist-lower-alpha{list-style-type:lower-alpha}.mw-parser-output .reflist-lower-greek{list-style-type:lower-greek}.mw-parser-output .reflist-lower-roman{list-style-type:lower-roman}</style><div class="reflist"> <div class="mw-references-wrap mw-references-columns"><ol class="references"> <li id="cite_note-MedicineNet-1"><span class="mw-cite-backlink"><b><a href="#cite_ref-MedicineNet_1-0">^</a></b></span> <span class="reference-text"><a rel="nofollow" class="external text" href="https://www.medterms.com/script/main/art.asp?articlekey=11428">MedicineNet > Definition of Twin</a> <a rel="nofollow" class="external text" href="https://web.archive.org/web/20131022143156/http://www.medterms.com/script/main/art.asp?articlekey=11428">Archived</a> 2013-10-22 at the <a href="/wiki/Wayback_Machine" title="Wayback Machine">Wayback Machine</a> Last Editorial Review: 19 June 2000</span> </li> <li id="cite_note-2"><span class="mw-cite-backlink"><b><a href="#cite_ref-2">^</a></b></span> <span class="reference-text">Michael R. 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Retrieved <span class="nowrap">8 January</span> 2021</span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=unknown&rft.jtitle=CNN&rft.atitle=Identical+twins+aren%27t+always+genetically+identical%2C+new+study+finds&rft.date=2021-01-08&rft.au=Lianne+Kolirin&rft_id=https%3A%2F%2Fwww.cnn.com%2F2021%2F01%2F08%2Fhealth%2Fidentical-twins-genome-study-scn-scli-intl%2F&rfr_id=info%3Asid%2Fen.wikipedia.org%3ATwin" class="Z3988"></span></span> </li> <li id="cite_note-41"><span class="mw-cite-backlink"><b><a href="#cite_ref-41">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFBieberWalter_E._NanceCynthia_C._MortonJudith_A._Brown1981" class="citation journal cs1">Bieber, Frederick R.; Walter E. Nance; Cynthia C. Morton; Judith A. Brown; Fay O. Redwine; Robert L. Jordan and T. Mohanakumar (1981-08-14). "Genetic Studies of an Acardiac Monster: Evidence of Polar Body Twinning in Man". <i>Science</i>. <b>213</b> (4509): 775–777. <a href="/wiki/Bibcode_(identifier)" class="mw-redirect" title="Bibcode (identifier)">Bibcode</a>:<a rel="nofollow" class="external text" href="https://ui.adsabs.harvard.edu/abs/1981Sci...213..775B">1981Sci...213..775B</a>. <a href="/wiki/Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1126%2Fscience.7196086">10.1126/science.7196086</a>. <a href="/wiki/JSTOR_(identifier)" class="mw-redirect" title="JSTOR (identifier)">JSTOR</a> <a rel="nofollow" class="external text" href="https://www.jstor.org/stable/1686613">1686613</a>. <a href="/wiki/PMID_(identifier)" class="mw-redirect" title="PMID (identifier)">PMID</a> <a rel="nofollow" class="external text" href="https://pubmed.ncbi.nlm.nih.gov/7196086">7196086</a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=article&rft.jtitle=Science&rft.atitle=Genetic+Studies+of+an+Acardiac+Monster%3A+Evidence+of+Polar+Body+Twinning+in+Man&rft.volume=213&rft.issue=4509&rft.pages=775-777&rft.date=1981-08-14&rft_id=info%3Adoi%2F10.1126%2Fscience.7196086&rft_id=info%3Apmid%2F7196086&rft_id=https%3A%2F%2Fwww.jstor.org%2Fstable%2F1686613%23id-name%3DJSTOR&rft_id=info%3Abibcode%2F1981Sci...213..775B&rft.aulast=Bieber&rft.aufirst=Frederick+R.&rft.au=Walter+E.+Nance&rft.au=Cynthia+C.+Morton&rft.au=Judith+A.+Brown&rft.au=Fay+O.+Redwine&rft.au=Robert+L.+Jordan+and+T.+Mohanakumar&rfr_id=info%3Asid%2Fen.wikipedia.org%3ATwin" class="Z3988"></span></span> </li> <li id="cite_note-42"><span class="mw-cite-backlink"><b><a href="#cite_ref-42">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFScottTreffStevensForman2012" class="citation journal cs1">Scott, Richard T.; Treff, Nathan R.; Stevens, John; Forman, Eric J.; Hong, Kathleen H.; Katz-Jaffe, Mandy G.; Schoolcraft, William B. 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Retrieved <span class="nowrap">7 February</span> 2022</span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=article&rft.jtitle=Nature&rft.atitle=%27Semi-identical%27+twins+discovered&rft.date=2007-03-26&rft_id=info%3Adoi%2F10.1038%2Fnews070326-1&rft_id=https%3A%2F%2Fapi.semanticscholar.org%2FCorpusID%3A85215225%23id-name%3DS2CID&rft.au=John+Whitfield&rft_id=https%3A%2F%2Fwww.nature.com%2Fnews%2F2007%2F070326%2Ffull%2Fnews070326-1.html&rfr_id=info%3Asid%2Fen.wikipedia.org%3ATwin" class="Z3988"></span></li></ul> </span></li> <li id="cite_note-44"><span class="mw-cite-backlink"><b><a href="#cite_ref-44">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://amp.cnn.com/cnn/2019/02/28/health/rare-twins-semi-identical-australia-trnd">"Semi-identical twins are rare, and doctors say they've identified the second case ever"</a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=Semi-identical+twins+are+rare%2C+and+doctors+say+they%27ve+identified+the+second+case+ever&rft_id=https%3A%2F%2Famp.cnn.com%2Fcnn%2F2019%2F02%2F28%2Fhealth%2Frare-twins-semi-identical-australia-trnd&rfr_id=info%3Asid%2Fen.wikipedia.org%3ATwin" class="Z3988"></span></span> </li> <li id="cite_note-45"><span class="mw-cite-backlink"><b><a href="#cite_ref-45">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFHall2003" class="citation journal cs1">Hall JG (2003). <a rel="nofollow" class="external text" href="https://web.archive.org/web/20160603021434/https://static.sdu.dk/mediafiles/7/8/D/%7B78DF6E68-11E3-47D4-A34F-9A086F1E9B1C%7Dhall2003.pdf">"Twinning"</a> <span class="cs1-format">(PDF)</span>. <i><a href="/wiki/The_Lancet" title="The Lancet">The Lancet</a></i>. <b>362</b> (9385): 735–43. <a href="/wiki/Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1016%2Fs0140-6736%2803%2914237-7">10.1016/s0140-6736(03)14237-7</a>. <a href="/wiki/PMID_(identifier)" class="mw-redirect" title="PMID (identifier)">PMID</a> <a rel="nofollow" class="external text" href="https://pubmed.ncbi.nlm.nih.gov/12957099">12957099</a>. <a href="/wiki/S2CID_(identifier)" class="mw-redirect" title="S2CID (identifier)">S2CID</a> <a rel="nofollow" class="external text" href="https://api.semanticscholar.org/CorpusID:208792233">208792233</a>. Archived from <a rel="nofollow" class="external text" href="https://static.sdu.dk/mediafiles/7/8/D/%7B78DF6E68-11E3-47D4-A34F-9A086F1E9B1C%7Dhall2003.pdf">the original</a> <span class="cs1-format">(PDF)</span> on 2016-06-03<span class="reference-accessdate">. Retrieved <span class="nowrap">2016-05-03</span></span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=article&rft.jtitle=The+Lancet&rft.atitle=Twinning&rft.volume=362&rft.issue=9385&rft.pages=735-43&rft.date=2003&rft_id=https%3A%2F%2Fapi.semanticscholar.org%2FCorpusID%3A208792233%23id-name%3DS2CID&rft_id=info%3Apmid%2F12957099&rft_id=info%3Adoi%2F10.1016%2Fs0140-6736%2803%2914237-7&rft.aulast=Hall&rft.aufirst=JG&rft_id=https%3A%2F%2Fstatic.sdu.dk%2Fmediafiles%2F7%2F8%2FD%2F%257B78DF6E68-11E3-47D4-A34F-9A086F1E9B1C%257Dhall2003.pdf&rfr_id=info%3Asid%2Fen.wikipedia.org%3ATwin" class="Z3988"></span></span> </li> <li id="cite_note-Shulman-46"><span class="mw-cite-backlink">^ <a href="#cite_ref-Shulman_46-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-Shulman_46-1"><sup><i><b>b</b></i></sup></a> <a href="#cite_ref-Shulman_46-2"><sup><i><b>c</b></i></sup></a> <a href="#cite_ref-Shulman_46-3"><sup><i><b>d</b></i></sup></a> <a href="#cite_ref-Shulman_46-4"><sup><i><b>e</b></i></sup></a> <a href="#cite_ref-Shulman_46-5"><sup><i><b>f</b></i></sup></a></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFShulmanvan_Vugt2006" class="citation book cs1">Shulman LS, van Vugt JG (2006). <span class="id-lock-limited" title="Free access subject to limited trial, subscription normally required"><a rel="nofollow" class="external text" href="https://archive.org/details/prenatalmedicine00john"><i>Prenatal medicine</i></a></span>. 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The excluded twin pregnancies were in cases of (1) delivery before 34 weeks of gestation; (2) when the first twin was delivered by <a href="/wiki/Caesarean_section" title="Caesarean section">caesarean section</a>; (3) when either of the twins had died in the womb before the onset of labor; and (4) when the pregnancy had been complicated by fetal malformations or <a href="/wiki/Twin-to-twin_transfusion_syndrome" title="Twin-to-twin transfusion syndrome">Twin-to-twin transfusion syndrome</a>.</span> </li> <li id="cite_note-69"><span class="mw-cite-backlink"><b><a href="#cite_ref-69">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFCrossland2019" class="citation news cs1">Crossland, David (16 April 2019). <span class="id-lock-subscription" title="Paid subscription required"><a rel="nofollow" class="external text" href="https://www.thetimes.co.uk/article/i-ll-do-it-my-way-twin-is-born-three-months-after-her-sister-f3jkjp72q">"I'll do it my way: twin is born three months after her sister"</a></span>. <i>The Times</i>. <a rel="nofollow" class="external text" href="https://web.archive.org/web/20210830143642/https://www.thetimes.co.uk/article/i-ll-do-it-my-way-twin-is-born-three-months-after-her-sister-f3jkjp72q">Archived</a> from the original on 30 August 2021<span class="reference-accessdate">. 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"Relationship between Twin Language, Twins' Bond, and Social Competence". <i>Twin Research and Human Genetics</i>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=article&rft.jtitle=Twin+Research+and+Human+Genetics&rft.atitle=Relationship+between+Twin+Language%2C+Twins%27+Bond%2C+and+Social+Competence&rft.date=2007&rft.aulast=Hayashi&rft.aufirst=C&rfr_id=info%3Asid%2Fen.wikipedia.org%3ATwin" class="Z3988"></span></span> </li> </ol></div></div> <div class="mw-heading mw-heading2"><h2 id="Further_reading">Further reading</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Twin&action=edit&section=37" title="Edit section: Further reading"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <ul><li>Bacon, Kate. <i>Twins in Society: Parents, Bodies, Space, and Talk</i> (Palgrave Macmillan; 2010) 221 pages; explores the experience of child twins, adult twins, and parents of twins, with a focus on Britain.</li> <li><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFNieuwintVan_Zalen-SprockHummelPals1999" class="citation journal cs1">Nieuwint, Aggie; Van Zalen-Sprock, Rieteke; Hummel, Pieter; Pals, Gerard; Van Vugt, John; et al. (1999). "<span class="cs1-kern-left"></span>'Identical' twins with discordant karyotypes". <i>Prenatal Diagnosis</i>. <b>19</b> (1): 72–6. <a href="/wiki/Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1002%2F%28SICI%291097-0223%28199901%2919%3A1%3C72%3A%3AAID-PD465%3E3.0.CO%3B2-V">10.1002/(SICI)1097-0223(199901)19:1<72::AID-PD465>3.0.CO;2-V</a>. <a href="/wiki/PMID_(identifier)" class="mw-redirect" title="PMID (identifier)">PMID</a> <a rel="nofollow" class="external text" href="https://pubmed.ncbi.nlm.nih.gov/10073913">10073913</a>. <a href="/wiki/S2CID_(identifier)" class="mw-redirect" title="S2CID (identifier)">S2CID</a> <a rel="nofollow" class="external text" href="https://api.semanticscholar.org/CorpusID:19112883">19112883</a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=article&rft.jtitle=Prenatal+Diagnosis&rft.atitle=%27Identical%27+twins+with+discordant+karyotypes&rft.volume=19&rft.issue=1&rft.pages=72-6&rft.date=1999&rft_id=https%3A%2F%2Fapi.semanticscholar.org%2FCorpusID%3A19112883%23id-name%3DS2CID&rft_id=info%3Apmid%2F10073913&rft_id=info%3Adoi%2F10.1002%2F%28SICI%291097-0223%28199901%2919%3A1%3C72%3A%3AAID-PD465%3E3.0.CO%3B2-V&rft.aulast=Nieuwint&rft.aufirst=Aggie&rft.au=Van+Zalen-Sprock%2C+Rieteke&rft.au=Hummel%2C+Pieter&rft.au=Pals%2C+Gerard&rft.au=Van+Vugt%2C+John&rft.au=Van+Der+Harten%2C+Hans&rft.au=Heins%2C+Yvonne&rft.au=Madan%2C+Kamlesh&rfr_id=info%3Asid%2Fen.wikipedia.org%3ATwin" class="Z3988"></span></li> <li><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFWenkHoutzBrooksChiafari1992" class="citation journal cs1">Wenk RE, Houtz T, Brooks M, Chiafari FA (1992). <a rel="nofollow" class="external text" href="https://doi.org/10.1017%2FS000156600000249X">"How frequent is heteropaternal superfecundation?"</a>. <i>Acta Geneticae Medicae et Gemellologiae</i>. <b>41</b> (1): 43–7. <a href="/wiki/Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<span class="id-lock-free" title="Freely accessible"><a rel="nofollow" class="external text" href="https://doi.org/10.1017%2FS000156600000249X">10.1017/S000156600000249X</a></span>. <a href="/wiki/PMID_(identifier)" class="mw-redirect" title="PMID (identifier)">PMID</a> <a rel="nofollow" class="external text" href="https://pubmed.ncbi.nlm.nih.gov/1488855">1488855</a>. <a href="/wiki/S2CID_(identifier)" class="mw-redirect" title="S2CID (identifier)">S2CID</a> <a rel="nofollow" class="external text" href="https://api.semanticscholar.org/CorpusID:23507167">23507167</a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=article&rft.jtitle=Acta+Geneticae+Medicae+et+Gemellologiae&rft.atitle=How+frequent+is+heteropaternal+superfecundation%3F&rft.volume=41&rft.issue=1&rft.pages=43-7&rft.date=1992&rft_id=https%3A%2F%2Fapi.semanticscholar.org%2FCorpusID%3A23507167%23id-name%3DS2CID&rft_id=info%3Apmid%2F1488855&rft_id=info%3Adoi%2F10.1017%2FS000156600000249X&rft.aulast=Wenk&rft.aufirst=RE&rft.au=Houtz%2C+T&rft.au=Brooks%2C+M&rft.au=Chiafari%2C+FA&rft_id=https%3A%2F%2Fdoi.org%2F10.1017%252FS000156600000249X&rfr_id=info%3Asid%2Fen.wikipedia.org%3ATwin" class="Z3988"></span></li> <li><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFGirelaLorenteAlvarezRodrigo1997" class="citation journal cs1">Girela, Eloy; Lorente, Jose A.; Alvarez, J. Carlos; Rodrigo, Maria D.; Lorente, Miguel; et al. (1997). "Indisputable double paternity in dizygous twins". <i>Fertility and Sterility</i>. <b>67</b> (6): 1159–61. <a href="/wiki/CiteSeerX_(identifier)" class="mw-redirect" title="CiteSeerX (identifier)">CiteSeerX</a> <span class="id-lock-free" title="Freely accessible"><a rel="nofollow" class="external text" href="https://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.378.4082">10.1.1.378.4082</a></span>. <a href="/wiki/Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1016%2FS0015-0282%2897%2981456-2">10.1016/S0015-0282(97)81456-2</a>. <a href="/wiki/PMID_(identifier)" class="mw-redirect" title="PMID (identifier)">PMID</a> <a rel="nofollow" class="external text" href="https://pubmed.ncbi.nlm.nih.gov/9176461">9176461</a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=article&rft.jtitle=Fertility+and+Sterility&rft.atitle=Indisputable+double+paternity+in+dizygous+twins&rft.volume=67&rft.issue=6&rft.pages=1159-61&rft.date=1997&rft_id=https%3A%2F%2Fciteseerx.ist.psu.edu%2Fviewdoc%2Fsummary%3Fdoi%3D10.1.1.378.4082%23id-name%3DCiteSeerX&rft_id=info%3Apmid%2F9176461&rft_id=info%3Adoi%2F10.1016%2FS0015-0282%2897%2981456-2&rft.aulast=Girela&rft.aufirst=Eloy&rft.au=Lorente%2C+Jose+A.&rft.au=Alvarez%2C+J.+Carlos&rft.au=Rodrigo%2C+Maria+D.&rft.au=Lorente%2C+Miguel&rft.au=Villanueva%2C+Enrique&rfr_id=info%3Asid%2Fen.wikipedia.org%3ATwin" class="Z3988"></span></li> <li><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFShinwellReichmanLerner-GevaBoyko2007" class="citation journal cs1">Shinwell ES, Reichman B, Lerner-Geva L, Boyko V, Blickstein I (September 2007). <a rel="nofollow" class="external text" href="https://pediatrics.aappublications.org/cgi/pmidlookup?view=long&pmid=17766488">"<span class="cs1-kern-left"></span>"Masculinizing" effect on respiratory morbidity in girls from unlike-sex preterm twins: a possible transchorionic paracrine effect"</a>. <i>Pediatrics</i>. <b>120</b> (3): e447–53. <a href="/wiki/Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1542%2Fpeds.2006-3574">10.1542/peds.2006-3574</a>. <a href="/wiki/PMID_(identifier)" class="mw-redirect" title="PMID (identifier)">PMID</a> <a rel="nofollow" class="external text" href="https://pubmed.ncbi.nlm.nih.gov/17766488">17766488</a>. <a href="/wiki/S2CID_(identifier)" class="mw-redirect" title="S2CID (identifier)">S2CID</a> <a rel="nofollow" class="external text" href="https://api.semanticscholar.org/CorpusID:20498737">20498737</a><span class="reference-accessdate">. Retrieved <span class="nowrap">2008-10-06</span></span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=article&rft.jtitle=Pediatrics&rft.atitle=%22Masculinizing%22+effect+on+respiratory+morbidity+in+girls+from+unlike-sex+preterm+twins%3A+a+possible+transchorionic+paracrine+effect&rft.volume=120&rft.issue=3&rft.pages=e447-53&rft.date=2007-09&rft_id=https%3A%2F%2Fapi.semanticscholar.org%2FCorpusID%3A20498737%23id-name%3DS2CID&rft_id=info%3Apmid%2F17766488&rft_id=info%3Adoi%2F10.1542%2Fpeds.2006-3574&rft.aulast=Shinwell&rft.aufirst=ES&rft.au=Reichman%2C+B&rft.au=Lerner-Geva%2C+L&rft.au=Boyko%2C+V&rft.au=Blickstein%2C+I&rft_id=https%3A%2F%2Fpediatrics.aappublications.org%2Fcgi%2Fpmidlookup%3Fview%3Dlong%26pmid%3D17766488&rfr_id=info%3Asid%2Fen.wikipedia.org%3ATwin" class="Z3988"></span></li> <li><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFLummaaPettayRussell2007" class="citation journal cs1">Lummaa V, Pettay JE, Russell AF (June 2007). <a rel="nofollow" class="external text" href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1904168">"Male twins reduce fitness of female co-twins in humans"</a>. <i>Proceedings of the National Academy of Sciences of the United States of America</i>. <b>104</b> (26): 10915–20. <a href="/wiki/Bibcode_(identifier)" class="mw-redirect" title="Bibcode (identifier)">Bibcode</a>:<a rel="nofollow" class="external text" href="https://ui.adsabs.harvard.edu/abs/2007PNAS..10410915L">2007PNAS..10410915L</a>. <a href="/wiki/Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<span class="id-lock-free" title="Freely accessible"><a rel="nofollow" class="external text" href="https://doi.org/10.1073%2Fpnas.0605875104">10.1073/pnas.0605875104</a></span>. <a href="/wiki/PMC_(identifier)" class="mw-redirect" title="PMC (identifier)">PMC</a> <span class="id-lock-free" title="Freely accessible"><a rel="nofollow" class="external text" href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1904168">1904168</a></span>. <a href="/wiki/PMID_(identifier)" class="mw-redirect" title="PMID (identifier)">PMID</a> <a rel="nofollow" class="external text" href="https://pubmed.ncbi.nlm.nih.gov/17576931">17576931</a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=article&rft.jtitle=Proceedings+of+the+National+Academy+of+Sciences+of+the+United+States+of+America&rft.atitle=Male+twins+reduce+fitness+of+female+co-twins+in+humans&rft.volume=104&rft.issue=26&rft.pages=10915-20&rft.date=2007-06&rft_id=https%3A%2F%2Fwww.ncbi.nlm.nih.gov%2Fpmc%2Farticles%2FPMC1904168%23id-name%3DPMC&rft_id=info%3Apmid%2F17576931&rft_id=info%3Adoi%2F10.1073%2Fpnas.0605875104&rft_id=info%3Abibcode%2F2007PNAS..10410915L&rft.aulast=Lummaa&rft.aufirst=V&rft.au=Pettay%2C+JE&rft.au=Russell%2C+AF&rft_id=https%3A%2F%2Fwww.ncbi.nlm.nih.gov%2Fpmc%2Farticles%2FPMC1904168&rfr_id=info%3Asid%2Fen.wikipedia.org%3ATwin" class="Z3988"></span></li> <li><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFScheinPaula_Bernstein2007" class="citation book cs1">Schein, Elyse; Paula Bernstein (2007). <i>Identical Strangers: A Memoir of Twins Separated and Reunited</i>. New York: Random House. <a href="/wiki/ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <a href="/wiki/Special:BookSources/978-1-4000-6496-0" title="Special:BookSources/978-1-4000-6496-0"><bdi>978-1-4000-6496-0</bdi></a>. <a href="/wiki/OCLC_(identifier)" class="mw-redirect" title="OCLC (identifier)">OCLC</a> <a rel="nofollow" class="external text" href="https://search.worldcat.org/oclc/123390922">123390922</a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=book&rft.btitle=Identical+Strangers%3A+A+Memoir+of+Twins+Separated+and+Reunited&rft.place=New+York&rft.pub=Random+House&rft.date=2007&rft_id=info%3Aoclcnum%2F123390922&rft.isbn=978-1-4000-6496-0&rft.aulast=Schein&rft.aufirst=Elyse&rft.au=Paula+Bernstein&rfr_id=info%3Asid%2Fen.wikipedia.org%3ATwin" class="Z3988"></span></li> <li><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFHelleVirpi_LummaaJukka_Jokela2004" class="citation journal cs1">Helle, Samuli; Virpi Lummaa; Jukka Jokela (2004). <a rel="nofollow" class="external text" href="https://web.archive.org/web/20081002173043/https://www.huli.group.shef.ac.uk/helleevolution2004.pdf">"Selection for Increased Brood Size in Historical Human Populations"</a> <span class="cs1-format">(PDF)</span>. <i>Evolution</i>. <b>58</b> (2): 430–436. <a href="/wiki/Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1111%2Fj.0014-3820.2004.tb01658.x">10.1111/j.0014-3820.2004.tb01658.x</a>. <a href="/wiki/PMID_(identifier)" class="mw-redirect" title="PMID (identifier)">PMID</a> <a rel="nofollow" class="external text" href="https://pubmed.ncbi.nlm.nih.gov/15068359">15068359</a>. <a href="/wiki/S2CID_(identifier)" class="mw-redirect" title="S2CID (identifier)">S2CID</a> <a rel="nofollow" class="external text" href="https://api.semanticscholar.org/CorpusID:9460009">9460009</a>. Archived from <a rel="nofollow" class="external text" href="https://www.huli.group.shef.ac.uk/helleevolution2004.pdf">the original</a> <span class="cs1-format">(PDF)</span> on 2008-10-02<span class="reference-accessdate">. Retrieved <span class="nowrap">2008-10-02</span></span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=article&rft.jtitle=Evolution&rft.atitle=Selection+for+Increased+Brood+Size+in+Historical+Human+Populations&rft.volume=58&rft.issue=2&rft.pages=430-436&rft.date=2004&rft_id=https%3A%2F%2Fapi.semanticscholar.org%2FCorpusID%3A9460009%23id-name%3DS2CID&rft_id=info%3Apmid%2F15068359&rft_id=info%3Adoi%2F10.1111%2Fj.0014-3820.2004.tb01658.x&rft.aulast=Helle&rft.aufirst=Samuli&rft.au=Virpi+Lummaa&rft.au=Jukka+Jokela&rft_id=https%3A%2F%2Fwww.huli.group.shef.ac.uk%2Fhelleevolution2004.pdf&rfr_id=info%3Asid%2Fen.wikipedia.org%3ATwin" class="Z3988"></span></li> <li><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://web.archive.org/web/20081030064734/https://www.twinsmagazine.com/media/images/Guide2008.pdf">"TWINS Guide to the First Year"</a> <span class="cs1-format">(PDF)</span>. <i>TWINS Magazine</i>. <a href="/wiki/Fort_Collins,_Colorado" title="Fort Collins, Colorado">Fort Collins, Colorado</a>. 2008. Archived from <a rel="nofollow" class="external text" href="https://www.twinsmagazine.com/media/images/Guide2008.pdf">the original</a> <span class="cs1-format">(PDF)</span> on 2008-10-30<span class="reference-accessdate">. Retrieved <span class="nowrap">2008-10-06</span></span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=unknown&rft.jtitle=TWINS+Magazine&rft.atitle=TWINS+Guide+to+the+First+Year&rft.date=2008&rft_id=https%3A%2F%2Fwww.twinsmagazine.com%2Fmedia%2Fimages%2FGuide2008.pdf&rfr_id=info%3Asid%2Fen.wikipedia.org%3ATwin" class="Z3988"></span></li> <li><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFSamson" class="citation web cs1">Samson, Jennifer. <a rel="nofollow" class="external text" href="http://arquivo.pt/wayback/20091015164305/https://www3.telus.net/tyee/multiples/index.html">"Facts About Multiples: An Encyclopedia of Multiple Birth Records"</a>. Archived from <a rel="nofollow" class="external text" href="https://www3.telus.net/tyee/multiples/index.html">the original</a> on 2009-10-15<span class="reference-accessdate">. Retrieved <span class="nowrap">2008-10-18</span></span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=Facts+About+Multiples%3A+An+Encyclopedia+of+Multiple+Birth+Records&rft.aulast=Samson&rft.aufirst=Jennifer&rft_id=https%3A%2F%2Fwww3.telus.net%2Ftyee%2Fmultiples%2Findex.html&rfr_id=info%3Asid%2Fen.wikipedia.org%3ATwin" class="Z3988"></span></li> <li><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFTaylor" class="citation web cs1">Taylor, Ally. <a rel="nofollow" class="external text" href="https://theblackandwhiteblog.com/twin-zygosity-test-for-dichorionic-diamniotic-di-di-twins-zygosity-reveal/">"Twin Zygosity Test for Dichorionic Diamniotic (Di/Di) Twins! Zygosity Reveal"</a><span class="reference-accessdate">. Retrieved <span class="nowrap">2018-03-30</span></span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=Twin+Zygosity+Test+for+Dichorionic+Diamniotic+%28Di%2FDi%29+Twins%21+Zygosity+Reveal&rft.aulast=Taylor&rft.aufirst=Ally&rft_id=https%3A%2F%2Ftheblackandwhiteblog.com%2Ftwin-zygosity-test-for-dichorionic-diamniotic-di-di-twins-zygosity-reveal%2F&rfr_id=info%3Asid%2Fen.wikipedia.org%3ATwin" class="Z3988"></span></li> <li>Am J Med Genet C Semin Med Genet. 2009 May 15;151C(2):136-41. Not really identical: epigenetic differences in monozygotic twins and implications for twin studies in psychiatry. Haque FN, Gottesman II, Wong AH.</li> <li>Twin brothers promoted as Majors General together. <a rel="nofollow" class="external text" href="https://www.dailymirror.lk/155256/Twin-brothers-promoted-as-Majors-General-together">Seneviratne brothers</a>, who are twins and joined the Army on a same day, were promoted to the rank of Major General, again on the same day.</li> <li><a rel="nofollow" class="external text" href="https://www.guinnessworldrecords.com/news/2022/11/theyre-so-different-even-though-theyre-identical-twins-725135">Guinness World Record holding twins: https://www.guinnessworldrecords.com/news/2022/11/theyre-so-different-even-though-theyre-identical-twins-725135</a></li></ul> <div class="mw-heading mw-heading2"><h2 id="External_links">External links</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Twin&action=edit&section=38" title="Edit section: External links"><span>edit</span></a><span 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srcset="//upload.wikimedia.org/wikipedia/en/thumb/4/4a/Commons-logo.svg/45px-Commons-logo.svg.png 1.5x, //upload.wikimedia.org/wikipedia/en/thumb/4/4a/Commons-logo.svg/59px-Commons-logo.svg.png 2x" data-file-width="1024" data-file-height="1376" /></span></span></div> <div class="side-box-text plainlist"><a href="/wiki/Wikimedia_Commons" title="Wikimedia Commons">Wikimedia Commons</a> has media related to:<br /> <a href="https://commons.wikimedia.org/wiki/Twin" class="extiw" title="commons:Twin"><span style="font-style:italic; font-weight:bold;">Twin</span></a> (<a href="https://commons.wikimedia.org/wiki/Category:Twins" class="extiw" title="commons:Category:Twins">category</a>)</div></div> </div> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1235681985"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1237033735"><div class="side-box side-box-right plainlinks sistersitebox"><link rel="mw-deduplicated-inline-style" 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1em;line-height:1.5em;text-align:center}.mw-parser-output .navbox-group{white-space:nowrap;text-align:right}.mw-parser-output .navbox,.mw-parser-output .navbox-subgroup{background-color:#fdfdfd}.mw-parser-output .navbox-list{line-height:1.5em;border-color:#fdfdfd}.mw-parser-output .navbox-list-with-group{text-align:left;border-left-width:2px;border-left-style:solid}.mw-parser-output tr+tr>.navbox-abovebelow,.mw-parser-output tr+tr>.navbox-group,.mw-parser-output tr+tr>.navbox-image,.mw-parser-output tr+tr>.navbox-list{border-top:2px solid #fdfdfd}.mw-parser-output .navbox-title{background-color:#ccf}.mw-parser-output .navbox-abovebelow,.mw-parser-output .navbox-group,.mw-parser-output .navbox-subgroup .navbox-title{background-color:#ddf}.mw-parser-output .navbox-subgroup .navbox-group,.mw-parser-output .navbox-subgroup .navbox-abovebelow{background-color:#e6e6ff}.mw-parser-output .navbox-even{background-color:#f7f7f7}.mw-parser-output .navbox-odd{background-color:transparent}.mw-parser-output .navbox .hlist td dl,.mw-parser-output .navbox .hlist td ol,.mw-parser-output .navbox .hlist td ul,.mw-parser-output .navbox td.hlist dl,.mw-parser-output .navbox td.hlist ol,.mw-parser-output .navbox td.hlist ul{padding:0.125em 0}.mw-parser-output .navbox .navbar{display:block;font-size:100%}.mw-parser-output .navbox-title .navbar{float:left;text-align:left;margin-right:0.5em}body.skin--responsive .mw-parser-output .navbox-image img{max-width:none!important}@media print{body.ns-0 .mw-parser-output .navbox{display:none!important}}</style></div><div role="navigation" class="navbox" aria-labelledby="Twin_conditions" style="padding:3px"><table class="nowraplinks mw-collapsible autocollapse navbox-inner" style="border-spacing:0;background:transparent;color:inherit"><tbody><tr><th scope="col" class="navbox-title" colspan="2"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1129693374"><style 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abbr{color:var(--color-base)!important}@media(prefers-color-scheme:dark){html.skin-theme-clientpref-os .mw-parser-output .navbar li a abbr{color:var(--color-base)!important}}@media print{.mw-parser-output .navbar{display:none!important}}</style><div class="navbar plainlinks hlist navbar-mini"><ul><li class="nv-view"><a href="/wiki/Template:Twin_conditions" title="Template:Twin conditions"><abbr title="View this template">v</abbr></a></li><li class="nv-talk"><a href="/wiki/Template_talk:Twin_conditions" title="Template talk:Twin conditions"><abbr title="Discuss this template">t</abbr></a></li><li class="nv-edit"><a href="/wiki/Special:EditPage/Template:Twin_conditions" title="Special:EditPage/Template:Twin conditions"><abbr title="Edit this template">e</abbr></a></li></ul></div><div id="Twin_conditions" style="font-size:114%;margin:0 4em"><a class="mw-selflink selflink">Twin</a> conditions</div></th></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="/wiki/Zygosity" title="Zygosity">Zygosity</a></th><td class="navbox-list-with-group navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a class="mw-selflink selflink">Twin</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="/wiki/Monochorionic_twins" title="Monochorionic twins">Monochorionic twins</a></th><td class="navbox-list-with-group navbox-list navbox-even hlist" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Twin-to-twin_transfusion_syndrome" title="Twin-to-twin transfusion syndrome">Twin-to-twin transfusion syndrome</a></li> <li><a href="/wiki/Twin_reversed_arterial_perfusion" title="Twin reversed arterial perfusion">Twin reversed arterial perfusion</a></li> <li><a href="/wiki/Monoamniotic_twins" title="Monoamniotic twins">Monoamniotic twins</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="/wiki/Conjoined_twins" title="Conjoined twins">Conjoined twins</a></th><td class="navbox-list-with-group navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Parasitic_twin" title="Parasitic twin">Parasitic twin</a> <ul><li><a href="/wiki/Craniopagus_parasiticus" title="Craniopagus parasiticus">Craniopagus parasiticus</a></li> <li><a href="/wiki/Fetus_in_fetu" title="Fetus in fetu">Fetus <i>in fetu</i></a></li></ul></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Other</th><td class="navbox-list-with-group navbox-list navbox-even hlist" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Vanishing_twin" title="Vanishing twin">Vanishing twin</a></li> <li><a href="/wiki/Chimera_(genetics)" title="Chimera (genetics)">Chimerism</a></li> <li><a href="/wiki/Mixed_twins" title="Mixed twins">Mixed twins</a></li></ul> </div></td></tr></tbody></table></div> <div class="navbox-styles"><link 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id="Authority_control_databases_frameless&#124;text-top&#124;10px&#124;alt=Edit_this_at_Wikidata&#124;link=https&#58;//www.wikidata.org/wiki/Q159979#identifiers&#124;class=noprint&#124;Edit_this_at_Wikidata" style="font-size:114%;margin:0 4em"><a href="/wiki/Help:Authority_control" title="Help:Authority control">Authority control databases</a> <span class="mw-valign-text-top noprint" typeof="mw:File/Frameless"><a href="https://www.wikidata.org/wiki/Q159979#identifiers" title="Edit this at Wikidata"><img alt="Edit this at Wikidata" src="//upload.wikimedia.org/wikipedia/en/thumb/8/8a/OOjs_UI_icon_edit-ltr-progressive.svg/10px-OOjs_UI_icon_edit-ltr-progressive.svg.png" decoding="async" width="10" height="10" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/en/thumb/8/8a/OOjs_UI_icon_edit-ltr-progressive.svg/15px-OOjs_UI_icon_edit-ltr-progressive.svg.png 1.5x, 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