In mammals, pregnancy begins when azygote (fertilized ovum)implants in the female'suterus and ends once the fetus leaves the uterus during labor or anabortion (whether induced or spontaneous).
In humans, pregnancy can be defined clinically, biochemically or biologically. Clinically, pregnancy starts from first day of the mother's last period.[4] Biochemically, pregnancy starts when a woman'shuman chorionic gonadotropin (hCG) levels rise above 25 mIU/mL.[5] 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.[6] Birth normally occurs at agestational age of about 40 weeks, though it is common for births to occur from 37 to 42 weeks.[6] Labor occurring prior to 37 weeks gestation is consideredpreterm labor and can result from multiple factors, including previous preterm deliveries.[7][8]
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.[9]
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.[10]
Various factors can influence the duration of gestation, includingdiseases in pregnancy and adequate prenatal care.[11] The rates of morbidity and pre-existing diseases that predispose mothers to life-threatening, pregnancy-related complications in the United States are increasing.[12] Inaccessibility of prenatal care may partially explain this ongoing disparity.[12][13]
During gestation inplacental mammals, there is a gradual physiological increase insenescence in the maternaldecidua (the specialized layer ofendometrium that forms the base of the placental bed) and inplacental cells.[14] This increase in senescence is associated with a gradual physiological increase inDNA damage during gestation. A positive correlation between the gestation period and maximum lifespan was observed across 740 mammalian species.[14] It was postulated that the rates of DNA damage and senescence may impact the gestation period as well as lifespan.[14]
In viviparousanimals, the embryo develops inside the body of the mother, as opposed to outside in anegg (oviparity). The mother then gives live birth. The less developed form of viviparity is calledovoviviparity, in which the mother carries embryos inside eggs. Mostvipers exhibit ovoviviparity.[15] The more developed form of viviparity is calledplacental viviparity; mammals are the best example, but it has also evolved independently in other animals, such as inscorpions, somesharks, and invelvet worms.[16][17][18] Viviparous offspring live independently and require an external food supply from birth. Certain lizards also employ this method such as thegeneraTiliqua andCorucia.[19][20] The placenta is attached directly to the mother in these lizards which is called viviparous matrotrophy.[21]
Ovoviviparousanimals develop withineggs 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, ovoviviparousembryos are nourished by theegg yolk rather than by the mother's body.[22] However, the mother's body does providegas exchange.[23] The young of ovoviviparousamphibians are sometimes born aslarvae, and undergometamorphosis outside the body of the mother.[24]
The fish familySyngnathidae has the unique characteristic whereby females lay theireggs in a brood pouch on the male's chest, and the male incubates the eggs.[25] Fertilization may take place in the pouch or before implantation in the water. Included in Syngnathidae areseahorses, thepipefish, and theweedy andleafy sea dragons.[26] Syngnathidae is the only family in the animal kingdom to which the term "male pregnancy" has been applied.[27]
^Keith L, Oleszczuk JJ (January 1999). "Iatrogenic multiple birth, multiple pregnancy and assisted reproductive technologies".International Journal of Gynaecology and Obstetrics.64 (1):11–25.doi:10.1016/S0020-7292(98)00230-6.PMID10190665.S2CID31814692.
^Liao JB, Buhimschi CS, Norwitz ER (June 2005). "Normal labor: mechanism and duration".Obstetrics and Gynecology Clinics of North America.32 (2):145–64, vii.doi:10.1016/j.ogc.2005.01.001.PMID15899352.
^Griggs KM, Hrelic DA, Williams N, McEwen-Campbell M, Cypher R (November 2020). "Preterm Labor and Birth: A Clinical Review".MCN: The American Journal of Maternal/Child Nursing.45 (6):328–337.doi:10.1097/NMC.0000000000000656.PMID33074911.S2CID224813648.
^Coley SL, Aronson RE (September 2013). "Exploring Birth Outcome Disparities and the Impact of Prenatal Care Utilization Among North Carolina Teen Mothers".Women's Health Issues.23 (5):e287 –e294.doi:10.1016/j.whi.2013.06.004.PMID23993476.
^abGadson, Alexis; Akpovi, Eloho; Mehta, Pooja K. (2017-08-01). "Exploring the social determinants of racial/ethnic disparities in prenatal care utilization and maternal outcome".Seminars in Perinatology.41 (5):308–317.doi:10.1053/j.semperi.2017.04.008.ISSN0146-0005.PMID28625554.
^Gadson A, Akpovi E, Mehta PK (August 2017). "Exploring the social determinants of racial/ethnic disparities in prenatal care utilization and maternal outcome".Seminars in Perinatology.41 (5):308–317.doi:10.1053/j.semperi.2017.04.008.PMID28625554.
^Neill WT (1964-01-01). "Viviparity in Snakes: Some Ecological and Zoogeographical Considerations".The American Naturalist.98 (898):35–55.doi:10.1086/282299.ISSN0003-0147.S2CID85209921.
^Thompson MB (December 2007). "Comparison of the respiratory transition at birth or hatching in viviparous and oviparous amniote vertebrates".Comparative Biochemistry and Physiology. Part A, Molecular & Integrative Physiology.148 (4):755–60.doi:10.1016/j.cbpa.2007.01.006.PMID17314056.
^Cook DF, Voss SC, Dadour IR (November 2012). "The laying of live larvae by the blowfly Calliphora varifrons (Diptera: Calliphoridae)".Forensic Science International.223 (1–3):44–6.doi:10.1016/j.forsciint.2012.07.015.PMID22921421.
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