gestation

{{main|Pregnancy (mammals)}}

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

{{main|Pregnancy}}

File:Human Fertilization.png

In humans, pregnancy can be defined clinically, biochemically or biologically. Clinically, pregnancy starts from first day of the mother's last period.{{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}} Biochemically, pregnancy starts when a woman's human chorionic gonadotropin (hCG) levels rise above 25 mIU/mL.{{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}} 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.{{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 }} Birth normally occurs at a gestational age of about 40 weeks, though it is common for births to occur from 37 to 42 weeks. Labor occurring prior to 37 weeks gestation is considered preterm labor and can result from multiple factors, including previous preterm deliveries.{{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 }}{{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}}

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 defects, maternal blood pressure, among others.{{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 }}

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.{{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}}

Various factors can influence the duration of gestation, including diseases in pregnancy and adequate prenatal care.{{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 }} The rates of morbidity and pre-existing diseases that predispose mothers to life-threatening, pregnancy-related complications in the United States are increasing.{{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 }} Inaccessibility of prenatal care may partially explain this ongoing disparity.{{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 }}

During gestation in 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 placental cells.{{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}} This increase in senescence is associated with a gradual physiological increase in DNA damage during gestation. A positive correlation between the gestation period and maximum lifespan was observed across 740 mammalian species. It was postulated that the rates of DNA damage and senescence may impact the gestation period as well as lifespan.

File:Pregnant scorpion.jpg

In viviparous animals, the embryo develops inside the body of the mother, as opposed to outside in an 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 vipers exhibit ovoviviparity.{{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}} The more developed form of viviparity is called placental viviparity; mammals are the best example, but it has also evolved independently in other animals, such as in scorpions, some sharks, and in velvet worms.{{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 }}{{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 }}{{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 }} Viviparous offspring live independently and require an external food supply from birth. Certain lizards also employ this method such as the genera Tiliqua and Corucia.{{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 }}{{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}} The placenta is attached directly to the mother in these lizards which is called viviparous matrotrophy.{{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 }}

Ovoviviparous animals develop within eggs 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 embryos are nourished by the egg yolk rather than by the mother's body.{{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 }} However, the mother's body does provide gas exchange.{{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 }} The young of ovoviviparous amphibians are sometimes born as larvae, and undergo metamorphosis outside the body of the mother.{{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 }}

The fish family Syngnathidae has the unique characteristic whereby females lay their eggs in a brood pouch on the male's chest, and the male incubates the eggs.{{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 }} Fertilization may take place in the pouch or before implantation in the water. Included in Syngnathidae are seahorses, the pipefish, and the weedy and leafy sea dragons.{{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 }} Syngnathidae is the only family in the animal kingdom to which the term "male pregnancy" has been applied.{{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 }}

• 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

{{Reflist}}

• [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