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<span>Placental circulation system</span> </div> </a> <ul id="toc-Placental_circulation_system-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Cellular_differentiation" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Cellular_differentiation"> <div class="vector-toc-text"> <span class="vector-toc-numb">2.3</span> <span>Cellular differentiation</span> </div> </a> <ul id="toc-Cellular_differentiation-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Gestation_periods" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Gestation_periods"> <div class="vector-toc-text"> <span class="vector-toc-numb">2.4</span> <span>Gestation periods</span> </div> </a> <ul id="toc-Gestation_periods-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-See_also" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#See_also"> <div class="vector-toc-text"> <span class="vector-toc-numb">3</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 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#References"> <div class="vector-toc-text"> <span class="vector-toc-numb">4</span> <span>References</span> </div> </a> <ul id="toc-References-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" 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mw-list-item"><a href="https://zh-min-nan.wikipedia.org/wiki/To%C3%A0-k%C5%ABi" title="Toà-kūi – Minnan" lang="nan" hreflang="nan" data-title="Toà-kūi" data-language-autonym="閩南語 / Bân-lâm-gú" data-language-local-name="Minnan" class="interlanguage-link-target"><span>閩南語 / Bân-lâm-gú</span></a></li><li class="interlanguage-link interwiki-cs mw-list-item"><a href="https://cs.wikipedia.org/wiki/B%C5%99ezost" title="Březost – Czech" lang="cs" hreflang="cs" data-title="Březost" data-language-autonym="Čeština" data-language-local-name="Czech" class="interlanguage-link-target"><span>Čeština</span></a></li><li class="interlanguage-link interwiki-de mw-list-item"><a href="https://de.wikipedia.org/wiki/Tr%C3%A4chtigkeit" title="Trächtigkeit – German" lang="de" hreflang="de" data-title="Trächtigkeit" 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/Tiinus" title="Tiinus – Estonian" lang="et" hreflang="et" data-title="Tiinus" data-language-autonym="Eesti" data-language-local-name="Estonian" class="interlanguage-link-target"><span>Eesti</span></a></li><li class="interlanguage-link interwiki-eo mw-list-item"><a href="https://eo.wikipedia.org/wiki/Mamula_gravedeco" title="Mamula gravedeco – Esperanto" lang="eo" hreflang="eo" data-title="Mamula gravedeco" data-language-autonym="Esperanto" data-language-local-name="Esperanto" class="interlanguage-link-target"><span>Esperanto</span></a></li><li class="interlanguage-link interwiki-fa mw-list-item"><a href="https://fa.wikipedia.org/wiki/%D8%A8%D8%A7%D8%B1%D8%AF%D8%A7%D8%B1%DB%8C_(%D9%BE%D8%B3%D8%AA%D8%A7%D9%86%D8%AF%D8%A7%D8%B1%D8%A7%D9%86)" title="بارداری (پستانداران) – Persian" lang="fa" hreflang="fa" data-title="بارداری (پستانداران)" data-language-autonym="فارسی" data-language-local-name="Persian" class="interlanguage-link-target"><span>فارسی</span></a></li><li class="interlanguage-link interwiki-hy mw-list-item"><a href="https://hy.wikipedia.org/wiki/%D5%80%D5%B2%D5%AB%D5%B8%D6%82%D5%A9%D5%B5%D5%B8%D6%82%D5%B6_(%D5%AF%D5%A5%D5%B6%D5%A4%D5%A1%D5%B6%D5%AB%D5%B6%D5%A5%D6%80)" title="Հղիություն (կենդանիներ) – Armenian" lang="hy" hreflang="hy" data-title="Հղիություն (կենդանիներ)" data-language-autonym="Հայերեն" data-language-local-name="Armenian" class="interlanguage-link-target"><span>Հայերեն</span></a></li><li class="interlanguage-link interwiki-ka mw-list-item"><a href="https://ka.wikipedia.org/wiki/%E1%83%9B%E1%83%90%E1%83%99%E1%83%94%E1%83%9D%E1%83%91%E1%83%90" title="მაკეობა – Georgian" lang="ka" hreflang="ka" data-title="მაკეობა" data-language-autonym="ქართული" data-language-local-name="Georgian" class="interlanguage-link-target"><span>ქართული</span></a></li><li class="interlanguage-link interwiki-ku mw-list-item"><a href="https://ku.wikipedia.org/wiki/Avis%C3%AE" 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Nynorsk" lang="nn" hreflang="nn" data-title="Drekt" 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-uz mw-list-item"><a href="https://uz.wikipedia.org/wiki/Bo%CA%BBg%CA%BBozlik" title="Boʻgʻozlik – Uzbek" lang="uz" hreflang="uz" data-title="Boʻgʻozlik" 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-kaa mw-list-item"><a href="https://kaa.wikipedia.org/wiki/Buwazl%C4%B1q" title="Buwazlıq – Kara-Kalpak" lang="kaa" hreflang="kaa" data-title="Buwazlıq" data-language-autonym="Qaraqalpaqsha" data-language-local-name="Kara-Kalpak" class="interlanguage-link-target"><span>Qaraqalpaqsha</span></a></li><li class="interlanguage-link interwiki-ru mw-list-item"><a href="https://ru.wikipedia.org/wiki/%D0%91%D0%B5%D1%80%D0%B5%D0%BC%D0%B5%D0%BD%D0%BD%D0%BE%D1%81%D1%82%D1%8C_%D0%BC%D0%BB%D0%B5%D0%BA%D0%BE%D0%BF%D0%B8%D1%82%D0%B0%D1%8E%D1%89%D0%B8%D1%85" title="Беременность млекопитающих – Russian" lang="ru" hreflang="ru" data-title="Беременность млекопитающих" data-language-autonym="Русский" data-language-local-name="Russian" class="interlanguage-link-target"><span>Русский</span></a></li><li class="interlanguage-link interwiki-fi mw-list-item"><a href="https://fi.wikipedia.org/wiki/Tiineys" title="Tiineys – Finnish" lang="fi" hreflang="fi" data-title="Tiineys" data-language-autonym="Suomi" data-language-local-name="Finnish" class="interlanguage-link-target"><span>Suomi</span></a></li><li class="interlanguage-link interwiki-tr mw-list-item"><a href="https://tr.wikipedia.org/wiki/Gebelik_(memeliler)" title="Gebelik (memeliler) – Turkish" lang="tr" hreflang="tr" data-title="Gebelik (memeliler)" data-language-autonym="Türkçe" 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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-More_citations_needed plainlinks metadata ambox ambox-content ambox-Refimprove" role="presentation"><tbody><tr><td class="mbox-image"><div class="mbox-image-div"><span typeof="mw:File"><a href="/wiki/File:Question_book-new.svg" class="mw-file-description"><img alt="" src="//upload.wikimedia.org/wikipedia/en/thumb/9/99/Question_book-new.svg/50px-Question_book-new.svg.png" decoding="async" width="50" height="39" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/en/thumb/9/99/Question_book-new.svg/75px-Question_book-new.svg.png 1.5x, //upload.wikimedia.org/wikipedia/en/thumb/9/99/Question_book-new.svg/100px-Question_book-new.svg.png 2x" data-file-width="512" data-file-height="399" /></a></span></div></td><td class="mbox-text"><div class="mbox-text-span">This article <b>needs additional citations for <a href="/wiki/Wikipedia:Verifiability" title="Wikipedia:Verifiability">verification</a></b>.<span class="hide-when-compact"> Please help <a href="/wiki/Special:EditPage/Pregnancy_(mammals)" title="Special:EditPage/Pregnancy (mammals)">improve this article</a> by <a href="/wiki/Help:Referencing_for_beginners" title="Help:Referencing for beginners">adding citations to reliable sources</a>. Unsourced material may be challenged and removed.<br /><small><span class="plainlinks"><i>Find sources:</i> <a rel="nofollow" class="external text" href="https://www.google.com/search?as_eq=wikipedia&q=%22Pregnancy%22+mammals">"Pregnancy" mammals</a> – <a rel="nofollow" class="external text" href="https://www.google.com/search?tbm=nws&q=%22Pregnancy%22+mammals+-wikipedia&tbs=ar:1">news</a> <b>·</b> <a rel="nofollow" class="external text" href="https://www.google.com/search?&q=%22Pregnancy%22+mammals&tbs=bkt:s&tbm=bks">newspapers</a> <b>·</b> <a rel="nofollow" class="external text" href="https://www.google.com/search?tbs=bks:1&q=%22Pregnancy%22+mammals+-wikipedia">books</a> <b>·</b> <a rel="nofollow" class="external text" href="https://scholar.google.com/scholar?q=%22Pregnancy%22+mammals">scholar</a> <b>·</b> <a rel="nofollow" class="external text" href="https://www.jstor.org/action/doBasicSearch?Query=%22Pregnancy%22+mammals&acc=on&wc=on">JSTOR</a></span></small></span> <span class="date-container"><i>(<span class="date">March 2012</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> <figure class="mw-halign-right" typeof="mw:File/Thumb"><a href="/wiki/File:PregnantLioness.jpg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/6/6d/PregnantLioness.jpg/300px-PregnantLioness.jpg" decoding="async" width="300" height="200" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/6/6d/PregnantLioness.jpg/450px-PregnantLioness.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/6/6d/PregnantLioness.jpg/600px-PregnantLioness.jpg 2x" data-file-width="2916" data-file-height="1947" /></a><figcaption>A pregnant <a href="/wiki/Lion" title="Lion">lioness</a>.</figcaption></figure> <p>In <a href="/wiki/Mammal" title="Mammal">mammals</a>, <b>pregnancy</b> is the period of <a href="/wiki/Reproduction" title="Reproduction">reproduction</a> during which a <a href="/wiki/Female" title="Female">female</a> carries one or more live offspring from <a href="/wiki/Implantation_(human_embryo)" class="mw-redirect" title="Implantation (human embryo)">implantation</a> in the uterus through <a href="/wiki/Gestation" title="Gestation">gestation</a>. It begins when a <a href="/wiki/Fertilisation" title="Fertilisation">fertilized</a> <a href="/wiki/Zygote" title="Zygote">zygote</a> implants in the female's <a href="/wiki/Uterus" title="Uterus">uterus</a>, and ends once it leaves the uterus. </p> <meta property="mw:PageProp/toc" /> <div class="mw-heading mw-heading2"><h2 id="Fertilization_and_implantation">Fertilization and implantation</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Pregnancy_(mammals)&action=edit&section=1" title="Edit section: Fertilization and implantation"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>During <a href="/wiki/Copulation_(zoology)" title="Copulation (zoology)">copulation</a>, the male <a href="/wiki/Insemination" title="Insemination">inseminates</a> the female. The <a href="/wiki/Spermatozoon" title="Spermatozoon">spermatozoon</a> <a href="/wiki/Fertilisation" title="Fertilisation">fertilizes</a> an <a href="/wiki/Ovum" class="mw-redirect" title="Ovum">ovum</a> or various ova in the uterus or <a href="/wiki/Oviduct" title="Oviduct">oviducts</a>, and this results in one or multiple <a href="/wiki/Zygote" title="Zygote">zygotes</a>. Sometimes, a zygote can be created by humans outside of the animal's body in the artificial process of <a href="/wiki/In_vitro_fertilisation" title="In vitro fertilisation">in-vitro fertilization</a>. After fertilization, the newly formed zygote then begins to divide through <a href="/wiki/Mitosis" title="Mitosis">mitosis</a>, forming an <a href="/wiki/Embryo" title="Embryo">embryo</a>, which implants in the female's <a href="/wiki/Endometrium" title="Endometrium">endometrium</a>. At this time, the embryo usually consists of 50 cells. </p> <div class="mw-heading mw-heading2"><h2 id="Development">Development</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Pregnancy_(mammals)&action=edit&section=2" title="Edit section: Development"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <figure class="mw-halign-left" typeof="mw:File/Thumb"><a href="/wiki/File:Elefetusus.jpg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/9/97/Elefetusus.jpg/500px-Elefetusus.jpg" decoding="async" width="500" height="354" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/9/97/Elefetusus.jpg/750px-Elefetusus.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/9/97/Elefetusus.jpg/1000px-Elefetusus.jpg 2x" data-file-width="1910" data-file-height="1352" /></a><figcaption>14 fetuses showing stages of embryo development in the African elephant</figcaption></figure> <div class="mw-heading mw-heading3"><h3 id="After_implantation">After implantation</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Pregnancy_(mammals)&action=edit&section=3" title="Edit section: After implantation"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>A blastocele is a small cavity on the center of the embryo, and the developing embryonary cells will grow around it. Then, a flat layer cell forms on the exterior of this cavity, and the <a href="/wiki/Zona_pellucida" title="Zona pellucida">zona pellucida</a>, the blastocyst's barrier, remains the same size as before. Cells grow increasingly smaller to fit in. This new structure with a cavity in the center and the developing cells around it is known as a <a href="/wiki/Blastocyst" title="Blastocyst">blastocyst</a>. </p><p>The presence of the blastocyst means that two types of cells are forming, an inner-cell mass growing on the interior of the blastocele and cells growing on the exterior of it. In 24 to 48 hours, the zona pellucida breaches. The cells on the exterior of the blastocyst begin excreting an enzyme which erodes <a href="/wiki/Epithelium" title="Epithelium">epithelial</a> uterine lining and creates a site for implantation. </p> <div class="mw-heading mw-heading3"><h3 id="Placental_circulation_system">Placental circulation system</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Pregnancy_(mammals)&action=edit&section=4" title="Edit section: Placental circulation system"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>The cells surrounding the blastocyst now destroy cells in the uterine lining, forming small pools of blood, which in turn stimulate the production of capillaries. This is the first stage in the growth of the <a href="/wiki/Placenta" title="Placenta">placenta</a>. The inner cell mass of the blastocyst divides rapidly, forming two layers. The top layer becomes the embryo, and cells from there occupy the <a href="/wiki/Amniotic_cavity" class="mw-redirect" title="Amniotic cavity">amniotic cavity</a>. At the same time, the bottom layer forms a small sac (if the cells begin developing in an abnormal position, an ectopic gestation may also occur at this point). </p><p>Several days later, <a href="/wiki/Chorionic_villi" title="Chorionic villi">chorionic villi</a> in the forming placenta anchor the implantation site to the uterus. A system of blood and <a href="/wiki/Blood_vessel" title="Blood vessel">blood vessels</a> now develops at the point of the newly forming placenta, growing near the implantation site. The small sac inside the blastocyst begins producing <a href="/wiki/Red_blood_cell" title="Red blood cell">red blood cells</a>. For the next 24 hours, <a href="/wiki/Connective_tissue" title="Connective tissue">connective tissue</a> develops between the developing placenta and the growing <a href="/wiki/Embryo" title="Embryo">embryo</a>. This later develops into the <a href="/wiki/Umbilical_cord" title="Umbilical cord">umbilical cord</a>. </p> <div class="mw-heading mw-heading3"><h3 id="Cellular_differentiation">Cellular differentiation</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Pregnancy_(mammals)&action=edit&section=5" title="Edit section: Cellular differentiation"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Following this, a narrow line of cells appears on the surface on the embryo. Its growth makes the embryo undergo <a href="/wiki/Gastrulation" title="Gastrulation">gastrulation</a>, in which the three <a href="/wiki/Germ_layer" title="Germ layer">primary tissue layers</a> of the fetus, the <a href="/wiki/Ectoderm" title="Ectoderm">ectoderm</a>, <a href="/wiki/Mesoderm" title="Mesoderm">mesoderm</a>, and <a href="/wiki/Endoderm" title="Endoderm">endoderm</a>, develop. The narrow line of cells begin to form the endoderm and mesoderm. The ectoderm begins to grow rapidly as a result of chemicals being produced by the mesoderm. These three layers give rise to all the various types of tissue in the body. </p><p>The endoderm later forms the lining of the <a href="/wiki/Tongue" title="Tongue">tongue</a>, <a href="/wiki/Gut_(anatomy)" class="mw-redirect" title="Gut (anatomy)">digestive tract</a>, <a href="/wiki/Lung" title="Lung">lungs</a>, <a href="/wiki/Urinary_bladder" class="mw-redirect" title="Urinary bladder">bladder</a> and several <a href="/wiki/Gland" title="Gland">glands</a>. The mesoderm forms <a href="/wiki/Muscle" title="Muscle">muscle</a>, <a href="/wiki/Bone" title="Bone">bone</a>, and <a href="/wiki/Lymph" title="Lymph">lymph</a> tissue, as well as the interior of the lungs, <a href="/wiki/Heart" title="Heart">heart</a>, and <a href="/wiki/Reproductive_system" title="Reproductive system">reproductive</a> and <a href="/wiki/Excretory_system" title="Excretory system">excretory systems</a>. It also gives rise to the <a href="/wiki/Spleen" title="Spleen">spleen</a>, and produces blood cells. The ectoderm forms the <a href="/wiki/Skin" title="Skin">skin</a>, <a href="/wiki/Nail_(anatomy)" title="Nail (anatomy)">nails</a>, <a href="/wiki/Hair" title="Hair">hair</a>, <a href="/wiki/Cornea" title="Cornea">cornea</a>, lining of the internal and external <a href="/wiki/Ear" title="Ear">ear</a>, <a href="/wiki/Nose" title="Nose">nose</a>, <a href="/wiki/Paranasal_sinuses" title="Paranasal sinuses">sinuses</a>, <a href="/wiki/Mouth" title="Mouth">mouth</a>, <a href="/wiki/Anus" title="Anus">anus</a>, <a href="/wiki/Tooth" title="Tooth">teeth</a>, <a href="/wiki/Pituitary_gland" title="Pituitary gland">pituitary gland</a>, <a href="/wiki/Mammary_gland" title="Mammary gland">mammary glands</a>, <a href="/wiki/Eye" title="Eye">eyes</a>, and all parts of the <a href="/wiki/Nervous_system" title="Nervous system">nervous system</a>. </p><p>Approximately 18 days after fertilization, the embryo has divided to form much of the tissue it will need. It is shaped like a pear, where the head region is larger than the tail. The embryo's nervous system is one of the first organic systems to grow. It begins growing in a concave area known as the <a href="/wiki/Neural_groove" title="Neural groove">neural groove</a>. </p><p>The blood system continues to grow networks which allow the blood to flow around the embryo. Blood cells are already being produced and are flowing through these developing networks. Secondary blood vessels also begin to develop around the placenta, to supply it with more nutrients. Blood cells begin to form on the sac in the center of the embryo, as well as cells which begin to <a href="/wiki/Cellular_differentiation" title="Cellular differentiation">differentiate</a> into blood vessels. <a href="/wiki/Endocardium" title="Endocardium">Endocardial</a> cells begin to form the <a href="/wiki/Cardiac_muscle" title="Cardiac muscle">myocardium</a>. </p><p>At about 24 days past fertilization, there is a primitive S-shaped tubule heart which begins beating. The flow of fluids throughout the embryo begins at this stage. </p> <div class="mw-heading mw-heading3"><h3 id="Gestation_periods">Gestation periods</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Pregnancy_(mammals)&action=edit&section=6" title="Edit section: Gestation periods"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></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">See also: <a href="/wiki/List_of_mammalian_gestation_durations" title="List of mammalian gestation durations">List of mammalian gestation durations</a></div> <p>For <a href="/wiki/Mammal" title="Mammal">mammals</a>, the <b><a href="/wiki/Gestation" title="Gestation">gestation</a> period</b> is the time in which a <a href="/wiki/Fetus" title="Fetus">fetus</a> develops, beginning with fertilization and ending at <a href="/wiki/Birth" title="Birth">birth</a>.<sup id="cite_ref-1" class="reference"><a href="#cite_note-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> The duration of this period varies between species. </p><p>For most species, the amount a <a href="/wiki/Fetus" title="Fetus">fetus</a> grows before birth determines the length of the gestation period. Smaller species normally have a shorter gestation period than larger animals.<sup id="cite_ref-britannica_2-0" class="reference"><a href="#cite_note-britannica-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> For example, <a href="/wiki/Cat#Reproduction" title="Cat">a cat's gestation</a> normally takes 58–65 days while <a href="/wiki/Elephant#Birth_and_development" title="Elephant">an elephant's</a> takes nearly 2 years (21 months).<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> However, growth does not necessarily determine the length of gestation for all species, especially for those with a <a href="/wiki/Breeding_season" class="mw-redirect" title="Breeding season">breeding season</a>. Species that use a breeding season usually give birth during a specific time of year when food is available.<sup id="cite_ref-britannica_2-1" class="reference"><a href="#cite_note-britannica-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> </p><p>Various other factors can come into play in determining the duration of gestation. For humans, male fetuses normally gestate several days longer than females and <a href="/wiki/Multiple_birth" title="Multiple birth">multiple pregnancies</a> gestate for a shorter period.<sup id="cite_ref-britannica_2-2" class="reference"><a href="#cite_note-britannica-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> Ethnicity in humans is also a factor that may lengthen or shorten gestation.<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> In dogs, there is a positive correlation between a longer gestation time and fewer members of the litter.<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 duration of gestation is usually longer in <a href="/wiki/Placental_mammal" class="mw-redirect" title="Placental mammal">placental mammals</a> than in <a href="/wiki/Marsupial" title="Marsupial">marsupials</a>.<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> </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=Pregnancy_(mammals)&action=edit&section=7" title="Edit section: See also"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <ul><li><a href="/wiki/Nesting_instinct" title="Nesting instinct">Nesting instinct</a></li> <li><a href="/wiki/Fetus_(biology)#Other_animals" class="mw-redirect" title="Fetus (biology)">Non-human fetuses</a></li> <li><a href="/wiki/Pseudopregnancy" title="Pseudopregnancy">Pseudopregnancy</a></li></ul> <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=Pregnancy_(mammals)&action=edit&section=8" 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 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