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==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 14thβ28/29th week, and the third trimester is 29/30thβ42nd 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 = 328β337 | 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 = 135β46 | 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=e287βe294|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=308β317|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 = 308β317 | 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/>
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