Gestation: Difference between revisions

[[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.|263x263px]]

[[File:Human Fertilization.png|alt=Timeline of human fertilization, ending with implantation of the blastocyst eight to nine days after fertilization.|thumb|364x364pxupright=1.5|Timeline of human fertilization]]

In humans, pregnancy can be defined clinically or biochemically. Clinically, pregnancy starts from first day of the mother's last missed 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>

[[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>

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> The brunt of this burden is experienced by non-Hispanic Black women. Inaccessibility of prenatal care may partially explain this ongoing disparity.<ref name="ReferenceA"/> Other factors that affect prenatal care utilization include socioeconomic status, insurance status, childcare, social support, housing, and immigration status.<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>

[[File:Pregnant scorpion.jpg|alt=Pregnant scorpion|thumb|Pregnant scorpion|195x195px]]

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 | 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 = httphttps://www.escholarship.org/uccontent/itemqt8z9122vw/8z9122vwqt8z9122vw.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 }}</ref>