Editing Gestation

{{Short description|Period during the carrying of an embryo}}
{{See also|Pregnancy}}
[[File:Feminology; a guide for womankind, giving in detail instructions as to motherhood, maidenhood, and the nursery (1902) (14761381441).jpg|alt=Drawing of a sagittal cross-section of a fetus in the pregnant parent's amniotic cavity.|thumb|upright|Drawing of a fetus in utero.]]
'''Gestation''' is the period of [[prenatal development|development]] during the carrying of an [[embryo]], and later [[fetus]], inside [[Viviparity|viviparous]] [[animals]] (the embryo develops within the parent).<ref>{{cite journal | vauthors = Mckay RJ, Lucey JF | title = NEONATOLOGY | journal = The New England Journal of Medicine | volume = 270 | issue = 23 | pages = 1231–6 CONTD | date = June 1964 | pmid = 14132827 | doi = 10.1056/NEJM196406042702306 }}</ref> It is typical for [[mammal]]s, but also occurs for some non-mammals. [[Pregnancy (mammals)|Mammals during pregnancy]] can have one or more gestations at the same time, for example in a [[multiple birth]].<ref>{{cite journal | vauthors = Keith L, Oleszczuk JJ | title = Iatrogenic multiple birth, multiple pregnancy and assisted reproductive technologies | journal = International Journal of Gynaecology and Obstetrics | volume = 64 | issue = 1 | pages = 11–25 | date = January 1999 | pmid = 10190665 | doi = 10.1016/S0020-7292(98)00230-6 | s2cid = 31814692 }}</ref>

The time interval of a gestation is called the ''[[gestation period]]''. In [[obstetrics and gynecology|obstetrics]], ''[[gestational age]]'' refers to the time since the onset of the last [[Menstruation#Onset and frequency)|menses]], which on average is [[Human fertilization#Fertilization age|fertilization age]] plus two weeks.<ref>{{cite journal | vauthors = Liao JB, Buhimschi CS, Norwitz ER | title = Normal labor: mechanism and duration | journal = Obstetrics and Gynecology Clinics of North America | volume = 32 | issue = 2 | pages = 145–64, vii | date = June 2005 | pmid = 15899352 | doi = 10.1016/j.ogc.2005.01.001 }}</ref>

==Mammals==
{{main|Pregnancy (mammals)}}

In mammals, pregnancy begins when a [[zygote]] (fertilized ovum) [[Implantation (embryology)|implants]] in the female's [[uterus]] and ends once the fetus leaves the uterus during labor or an [[abortion]] (whether induced or spontaneous).

===Humans===
{{main|Pregnancy}}
[[File:Human Fertilization.png|alt=Timeline of human fertilization, ending with implantation of the blastocyst eight to nine days after fertilization.|thumb|upright=1.5|Timeline of human fertilization]]
In humans, pregnancy can be defined clinically, biochemically or biologically. Clinically, pregnancy starts from first day of the mother's last period.<ref>{{cite web |title=You and your baby at 0-8 weeks pregnant |url=https://111.wales.nhs.uk/LiveWell/Pregnancy/4to8weeks/ |website=NHS 111 Wales |date=26 April 2018 |publisher=NHS Wales |access-date=24 December 2022}}</ref> Biochemically, pregnancy starts when a woman's [[human chorionic gonadotropin]] (hCG) levels rise above 25 mIU/mL.<ref>{{Cite web|date=2020-04-26|title=What is HCG?|url=https://americanpregnancy.org/getting-pregnant/hcg-levels/|access-date=2021-09-13|website=American Pregnancy Association|language=en-US}}</ref> Biologically, pregnancy starts at implantation of the fertilized egg.

Human pregnancy can be divided into three trimesters, each approximately three months long: the first, second, and third trimester. The first trimester is from the last menstrual period through the 13th week, the second trimester is 14th–28/29th week, and the third trimester is 29/30th–42nd week.<ref name="ACOG_BabyGrowth">{{cite web | url = http://www.acog.org/~/media/For%20Patients/faq156.pdf?dmc=1&ts=20130303T2230398164 | publisher = American College of Obstetricians and Gynecologists | title = How Your Baby Grows During Pregnancy }}</ref> Birth normally occurs at a [[Gestational age (obstetrics)|gestational age]] of about 40 weeks, though it is common for births to occur from 37 to 42 weeks.<ref name="ACOG_BabyGrowth" /> Labor occurring prior to 37 weeks gestation is considered [[Preterm birth|preterm labor]] and can result from multiple factors, including previous preterm deliveries.<ref>{{cite journal | vauthors = Griggs KM, Hrelic DA, Williams N, McEwen-Campbell M, Cypher R | title = Preterm Labor and Birth: A Clinical Review | journal = MCN: The American Journal of Maternal/Child Nursing | volume = 45 | issue = 6 | pages = 328–337 | date = November 2020 | pmid = 33074911 | doi = 10.1097/NMC.0000000000000656 | s2cid = 224813648 }}</ref><ref>{{Cite web|title=Preterm Labor and Birth|url=https://www.acog.org/en/womens-health/faqs/preterm-labor-and-birth|access-date=2021-09-13|website=www.acog.org|language=en}}</ref>

[[Prenatal care]] is important for the maintenance of a healthy pregnancy and surveillance of related complications. In high-income countries, prenatal care typically involves monthly visits during the first two trimesters, with an increasing number of visits closer to delivery. At these visits, healthcare providers will evaluate a variety of parental and fetal metrics, including fetal growth and heart rate, [[birth defect]]s, maternal blood pressure, among others.<ref>{{cite journal | vauthors = Helton MR | title = Prenatal care | journal = Primary Care | volume = 24 | issue = 1 | pages = 135–46 | date = March 1997 | doi = 10.1016/S0095-4543(22)00091-4 | pmid = 9016732 | s2cid = 36322807 }}</ref>

After birth, health care providers will measure the baby's weight, vital signs, reflexes, head circumference, muscle tone, and posture to help determine the gestational age.<ref>{{Cite web|title=Gestational age: MedlinePlus Medical Encyclopedia|url=https://medlineplus.gov/ency/article/002367.htm|access-date=2021-09-13|website=medlineplus.gov|language=en}}</ref>

Various factors can influence the duration of gestation, including [[diseases in pregnancy]] and adequate prenatal care.<ref>{{Cite journal| vauthors = Coley SL, Aronson RE |date= September 2013 |title= Exploring Birth Outcome Disparities and the Impact of Prenatal Care Utilization Among North Carolina Teen Mothers |journal=Women's Health Issues|language=en|volume=23|issue=5|pages=e287–e294|doi=10.1016/j.whi.2013.06.004|pmid= 23993476 }}</ref> The rates of morbidity and pre-existing diseases that predispose mothers to life-threatening, pregnancy-related complications in the United States are increasing.<ref name="ReferenceA">{{Cite journal|date=2017-08-01|title=Exploring the social determinants of racial/ethnic disparities in prenatal care utilization and maternal outcome |journal=Seminars in Perinatology|language=en|volume=41|issue=5|pages=308–317|doi=10.1053/j.semperi.2017.04.008|issn=0146-0005|last1=Gadson |first1=Alexis |last2=Akpovi |first2=Eloho |last3=Mehta |first3=Pooja K. |pmid=28625554 }}</ref> Inaccessibility of prenatal care may partially explain this ongoing disparity.<ref name="ReferenceA"/><ref>{{cite journal | vauthors = Gadson A, Akpovi E, Mehta PK | title = Exploring the social determinants of racial/ethnic disparities in prenatal care utilization and maternal outcome | journal = Seminars in Perinatology | volume = 41 | issue = 5 | pages = 308–317 | date = August 2017 | pmid = 28625554 | doi = 10.1053/j.semperi.2017.04.008 }}</ref>

===Placental mammals===

During gestation in [[placentalia|placental mammals]], there is a gradual physiological increase in [[senescence]] in the maternal [[decidua]] (the specialized layer of [[endometrium]] that forms the base of the placental bed) and in [[placenta]]l cells.<ref name = Singh2024>{{cite journal |vauthors=Singh VP, Singh P |title=Linking DNA damage and senescence to gestation period and lifespan in placental mammals |journal=Front Cell Dev Biol |volume=12 |issue= |pages=1480695 |date=2024 |pmid=39403126 |pmc=11471632 |doi=10.3389/fcell.2024.1480695 |doi-access=free |url=}}{{Creative Commons text attribution notice|cc=by4|from this source=yes}}</ref>  This increase in senescence is associated with a gradual physiological increase in [[DNA damage (naturally occurring)|DNA damage]] during gestation. A positive correlation between the gestation period and maximum lifespan was observed across 740 mammalian species.<ref name = Singh2024/>  It was postulated that the rates of DNA damage and senescence may impact the gestation period as well as lifespan.<ref name = Singh2024/>

==Non-mammals==
[[File:Pregnant scorpion.jpg|alt=Pregnant scorpion|thumb|Pregnant scorpion]]
In viviparous [[animal]]s, the embryo develops inside the body of the mother, as opposed to outside in an [[Egg (biology)|egg]] ([[oviparity]]). The mother then gives live birth. The less developed form of viviparity is called [[ovoviviparity]], in which the mother carries embryos inside eggs. Most [[Viperidae|vipers]] exhibit ovoviviparity.<ref>{{Cite journal| vauthors = Neill WT |date=1964-01-01|title=Viviparity in Snakes: Some Ecological and Zoogeographical Considerations |journal=The American Naturalist|volume=98|issue=898|pages=35–55|doi=10.1086/282299|s2cid=85209921|issn=0003-0147}}</ref> The more developed form of viviparity is called [[placenta|placental viviparity]]; mammals are the best example, but it has also evolved independently in other animals, such as in [[scorpion]]s, some [[shark]]s, and in [[velvet worm]]s.<ref>{{cite journal | vauthors = Bainbridge DR | title = The evolution of pregnancy | journal = Early Human Development | volume = 90 | issue = 11 | pages = 741–5 | date = November 2014 | pmid = 25242206 | doi = 10.1016/j.earlhumdev.2014.08.013 }}</ref><ref>{{cite journal | vauthors = Carter AM, Soma H | title = Viviparity in the longest-living vertebrate, the Greenland shark (Somniosus microcephalus) | journal = Placenta | volume = 97 | pages = 26–28 | date = August 2020 | pmid = 32792058 | doi = 10.1016/j.placenta.2020.05.014 | s2cid = 221121663 }}</ref><ref>{{cite journal | vauthors = Smith MR | title = Evolution: Velvet Worm Biogeography | journal = Current Biology | volume = 26 | issue = 19 | pages = R882–R884 | date = October 2016 | pmid = 27728789 | doi = 10.1016/j.cub.2016.07.067 | s2cid = 4039461 | doi-access = free | bibcode = 2016CBio...26.R882S | url = https://durham-repository.worktribe.com/preview/1377545/19346.pdf }}</ref> Viviparous offspring live independently and require an external food supply from birth. Certain lizards also employ this method such as the [[Genus|genera]] ''[[Blue-tongued skink|Tiliqua]]'' and ''[[Solomon Islands skink|Corucia]].''<ref>{{cite journal | vauthors = Munns SL, Edwards A, Nicol S, Frappell PB | title = Pregnancy limits lung function during exercise and depresses metabolic rate in the skink Tiliqua nigrolutea | journal = The Journal of Experimental Biology | volume = 218 | issue = Pt 6 | pages = 931–9 | date = March 2015 | pmid = 25788728 | doi = 10.1242/jeb.111450 | s2cid = 16426853 | doi-access = free }}</ref><ref>{{Cite web | vauthors = Hutchin K |date=2021-07-19|title=Prehensile-tailed Skink|url=https://aaric.org/2021/07/19/prehensile-tailed-skink/|access-date=2021-09-19|website=Ambassador Animal|language=en}}</ref> The placenta is attached directly to the mother in these lizards which is called viviparous matrotrophy.<ref>{{cite journal | vauthors = Ostrovsky AN, Lidgard S, Gordon DP, Schwaha T, Genikhovich G, Ereskovsky AV | title = Matrotrophy and placentation in invertebrates: a new paradigm | journal = Biological Reviews of the Cambridge Philosophical Society | volume = 91 | issue = 3 | pages = 673–711 | date = August 2016 | pmid = 25925633 | pmc = 5098176 | doi = 10.1111/brv.12189 }}</ref>

Ovoviviparous [[animal]]s develop within [[Egg (biology)|egg]]s that remain within the mother's body up until they hatch or are about to hatch. It is similar to viviparity in that the embryo develops within the mother's body. Unlike the embryos of viviparous species, ovoviviparous [[embryo]]s are nourished by the [[egg yolk]] rather than by the mother's body.<ref>{{cite journal | vauthors = Mueller LD, Bitner K | title = The Evolution of Ovoviviparity in a Temporally Varying Environment | journal = The American Naturalist | volume = 186 | issue = 6 | pages = 708–15 | date = December 2015 | pmid = 26655978 | doi = 10.1086/683661 | s2cid = 7447706 | url = https://escholarship.org/content/qt8z9122vw/qt8z9122vw.pdf?t=pgoz6f }}</ref> However, the mother's body does provide [[gas exchange]].<ref>{{cite journal | vauthors = Thompson MB | title = Comparison of the respiratory transition at birth or hatching in viviparous and oviparous amniote vertebrates | journal = Comparative Biochemistry and Physiology. Part A, Molecular & Integrative Physiology | volume = 148 | issue = 4 | pages = 755–60 | date = December 2007 | pmid = 17314056 | doi = 10.1016/j.cbpa.2007.01.006 }}</ref> The young of ovoviviparous [[amphibian]]s are sometimes born as [[larva]]e, and undergo [[Metamorphosis (biology)|metamorphosis]] outside the body of the mother.<ref>{{cite journal | vauthors = Cook DF, Voss SC, Dadour IR | title = The laying of live larvae by the blowfly Calliphora varifrons (Diptera: Calliphoridae) | journal = Forensic Science International | volume = 223 | issue = 1–3 | pages = 44–6 | date = November 2012 | pmid = 22921421 | doi = 10.1016/j.forsciint.2012.07.015 }}</ref>

The fish family [[Syngnathidae]] has the unique characteristic whereby females lay their [[egg (biology)|eggs]] in a brood pouch on the male's chest, and the male incubates the eggs.<ref>{{cite journal | vauthors = Scobell SK, Mackenzie DS | title = Reproductive endocrinology of Syngnathidae | journal = Journal of Fish Biology | volume = 78 | issue = 6 | pages = 1662–80 | date = June 2011 | pmid = 21651522 | doi = 10.1111/j.1095-8649.2011.02994.x | doi-access = free | bibcode = 2011JFBio..78.1662S }}</ref> Fertilization may take place in the pouch or before implantation in the water. Included in Syngnathidae are [[seahorse]]s, the [[pipefish]], and the [[weedy sea dragon|weedy]] and [[leafy sea dragon]]s.<ref>{{cite journal | vauthors = Wilson AB, Orr JW | title = The evolutionary origins of Syngnathidae: pipefishes and seahorses | journal = Journal of Fish Biology | volume = 78 | issue = 6 | pages = 1603–23 | date = June 2011 | pmid = 21651519 | doi = 10.1111/j.1095-8649.2011.02988.x | bibcode = 2011JFBio..78.1603W | url = http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1330&context=usdeptcommercepub }}</ref> Syngnathidae is the only family in the animal kingdom to which the term "[[male pregnancy]]" has been applied.<ref name="biology">{{cite journal | vauthors = Jones AG, Avise JC | title = Male pregnancy | journal = Current Biology | volume = 13 | issue = 20 | pages = R791 | date = October 2003 | pmid = 14561416 | doi = 10.1016/j.cub.2003.09.045 | doi-access = free | bibcode = 2003CBio...13.R791J }}</ref>

== See also ==
*[[Evolution of mammals]] (for the evolution of gestation-related features in humans and other mammals)
*[[Male pregnancy]]
*[[Nesting instinct]]
*[[Pregnancy]]
*[[Pregnancy in fish]]
*[[Prenatal development]]
*[[Prenatal nutrition and birth weight]]

== References ==
{{Reflist}}

== External links ==
* [https://web.archive.org/web/20100305061829/http://www.nsrl.ttu.edu/tmot1/mus_musc.htm NSRL.ttu.edu]
* [http://www.infoplease.com/ipa/A0004723.html Table listing average animal gestation periods]

[[Category:Reproduction in mammals]]