Editing Placenta (section)

==Physiology==

===Development===
[[File:Placenta.png|thumb|Placenta]]
[[File:HumanEmbryogenesis.svg|thumb|300px|The initial stages of [[human embryogenesis]]]]
{{Further|Placentation}}
The placenta begins to develop upon [[Implantation (embryology)|implantation]] of the [[blastocyst]] into the maternal [[endometrium]], very early on in pregnancy at about week 4.<ref name="Merck">{{Cite web |title=Stages of Development of the Fetus - Women's Health Issues |url=https://www.merckmanuals.com/home/women-s-health-issues/normal-pregnancy/stages-of-development-of-the-fetus |access-date=2022-06-12 |website=Merck Manuals Consumer Version |language=en-US}}</ref>

The outer layer of the late blastocyst, is formed of [[trophoblast]]s, cells that form the outer layer of the placenta. This outer layer is divided into two further layers: the underlying [[cytotrophoblast]] layer and the overlying [[syncytiotrophoblast]] layer. The syncytiotrophoblast is a [[multinucleate]]d continuous cell layer that covers the surface of the placenta. It forms as a result of differentiation and fusion of the underlying cytotrophoblasts, a process that continues throughout placental development. The syncytiotrophoblast contributes to the barrier function of the placenta.<ref>{{Cite web |title=How Your Fetus Grows During Pregnancy |url=https://www.acog.org/en/womens-health/faqs/how-your-fetus-grows-during-pregnancy |access-date=2022-06-12 |website=www.acog.org |language=en}}</ref>

The placenta grows throughout [[pregnancy]]. Development of the maternal blood supply to the placenta is complete by the end of the first trimester of pregnancy week 14 (DM).<ref name="Merck" />

===Placental circulation===
[[File:Gray39.png|thumb|Maternal blood fills the [[intervillous space]], nutrients, water, and gases are actively and passively exchanged, then deoxygenated blood is displaced by the next maternal pulse.]]

====Maternal placental circulation====
In preparation for implantation of the blastocyst, the endometrium undergoes [[decidualization]]. [[Spiral arteries]] in the [[decidua]] are remodeled so that they become less convoluted and their diameter is increased. The increased diameter and straighter flow path both act to increase maternal blood flow to the placenta. There is relatively high pressure as the maternal blood fills [[intervillous space]] through these spiral arteries which bathe the fetal [[Chorionic villi|villi]] in blood, allowing an exchange of gases to take place. In humans and other hemochorial placentals, the maternal blood comes into direct contact with the fetal [[chorion]], though no fluid is exchanged. As the pressure decreases between [[pulse]]s, the deoxygenated blood flows back through the endometrial veins.{{citation needed|date=September 2023}}

Maternal blood flow begins between days 5–12,<ref>{{cite book | vauthors = Dashe JS, Bloom SL, Spong CY, Hoffman BL |title=Williams Obstetrics |date=2018 |publisher=McGraw Hill Professional |isbn=978-1-259-64433-7 }}{{page needed|date=August 2021}}</ref> and is approximately 600–700&nbsp;ml/min at term.

====Fetoplacental circulation====
{{Further|Fetal circulation}}
Deoxygenated fetal blood passes through [[umbilical arteries]] to the placenta. At the junction of umbilical cord and placenta, the umbilical arteries branch radially to form [[chorionic arteries]]. Chorionic arteries, in turn, branch into [[cotyledon arteries]]. In the villi, these vessels eventually branch to form an extensive arterio-capillary-venous system, bringing the fetal blood extremely close to the maternal blood; but no intermingling of fetal and maternal blood occurs ("placental barrier").<ref>{{cite web | url = http://www.embryology.ch/anglais/fplacenta/circulplac01.html#placentaire | title = Placental blood circulation | archive-url = https://web.archive.org/web/20110928163956/http://www.embryology.ch/anglais/fplacenta/circulplac01.html | archive-date=2011-09-28 | work = Online course in embryology for medical students | publisher = Universities of Fribourg, Lausanne and Bern (Switzerland) }}</ref>

[[Endothelin]] and [[prostanoid]]s cause [[vasoconstriction]] in placental arteries, while [[nitric oxide]] causes [[vasodilation]].<ref name=kiserud2004/> On the other hand, there is no neural vascular regulation, and catecholamines have only little effect.<ref name=kiserud2004>{{cite journal | vauthors = Kiserud T, Acharya G | title = The fetal circulation | journal = Prenatal Diagnosis | volume = 24 | issue = 13 | pages = 1049–1059 | date = December 2004 | pmid = 15614842 | doi = 10.1002/pd.1062 | s2cid = 25040285 }}</ref>

The fetoplacental circulation is vulnerable to persistent hypoxia or intermittent hypoxia and reoxygenation, which can lead to generation of excessive [[free radical]]s. This may contribute to [[pre-eclampsia]] and other [[pregnancy complications]].<ref name="ReiterTan2013">{{cite journal | vauthors = Reiter RJ, Tan DX, Korkmaz A, Rosales-Corral SA | title = Melatonin and stable circadian rhythms optimize maternal, placental and fetal physiology | journal = Human Reproduction Update | volume = 20 | issue = 2 | pages = 293–307 | year = 2013 | pmid = 24132226 | doi = 10.1093/humupd/dmt054 | doi-access = free }}</ref> It is proposed that [[melatonin]] plays a role as an [[antioxidant]] in the placenta.<ref name="ReiterTan2013"/>

This begins at day 17–22.<ref>{{Cite book|title=Williams book of obsteritcis}}</ref>

===Birth===
{{Main|Placental expulsion}}
[[Placental expulsion]] begins as a physiological separation from the wall of the uterus. The period from just after the child is born until just after the placenta is expelled is called the "third stage of labor". 

Placental expulsion can be managed actively, for example by giving [[oxytocin]] via intramuscular injection followed by cord traction to assist in delivering the placenta. Alternatively, it can be managed expectantly, allowing the placenta to be expelled without medical assistance. Blood loss and the risk of [[postpartum bleeding]] may be reduced in women offered active management of the third stage of labour, however there may be adverse effects and more research is necessary.<ref>{{cite journal | vauthors = Begley CM, Gyte GM, Devane D, McGuire W, Weeks A, Biesty LM | title = Active versus expectant management for women in the third stage of labour | journal = The Cochrane Database of Systematic Reviews | volume = 2019 | issue = 2 | pages = CD007412 | date = February 2019 | pmid = 30754073 | pmc = 6372362 | doi = 10.1002/14651858.CD007412.pub5 }}</ref>

The habit is to cut the cord immediately after birth, but it may be no medical reason to do this; on the contrary, not cutting the cord could sometimes help the baby in its [[adaptation to extrauterine life]], for preterm infants.<ref>{{cite journal | vauthors = Mercer JS, Vohr BR, Erickson-Owens DA, Padbury JF, Oh W | title = Seven-month developmental outcomes of very low birth weight infants enrolled in a randomized controlled trial of delayed versus immediate cord clamping | journal = Journal of Perinatology | volume = 30 | issue = 1 | pages = 11–16 | date = January 2010 | pmid = 19847185 | pmc = 2799542 | doi = 10.1038/jp.2009.170 }}</ref>

===Microbiome===
{{main|Placental microbiome}}
The placenta is traditionally thought to be [[Asepsis|sterile]], but recent research suggests that a resident, [[Nonpathogenic organisms|non-pathogenic]], and diverse population of [[microorganisms]] may be present in healthy tissue. However, whether these microbes exist or are clinically important is highly controversial and is the subject of active research.<ref name="Per2017">{{cite journal | vauthors = Perez-Muñoz ME, Arrieta MC, Ramer-Tait AE, Walter J | title = A critical assessment of the "sterile womb" and "in utero colonization" hypotheses: implications for research on the pioneer infant microbiome | journal = Microbiome | volume = 5 | issue = 1 | pages = 48 | date = April 2017 | pmid = 28454555 | pmc = 5410102 | doi = 10.1186/s40168-017-0268-4 | doi-access = free }}</ref><ref name="MorKwon2015">{{cite journal | vauthors = Mor G, Kwon JY | title = Trophoblast-microbiome interaction: a new paradigm on immune regulation | journal = American Journal of Obstetrics and Gynecology | volume = 213 | issue = 4 Suppl | pages = S131–S137 | date = October 2015 | pmid = 26428492 | pmc = 6800181 | doi = 10.1016/j.ajog.2015.06.039 }}</ref><ref name="PrinceAntony2014">{{cite journal | vauthors = Prince AL, Antony KM, Chu DM, Aagaard KM | title = The microbiome, parturition, and timing of birth: more questions than answers | journal = Journal of Reproductive Immunology | volume = 104-105 | pages = 12–19 | date = October 2014 | pmid = 24793619 | pmc = 4157949 | doi = 10.1016/j.jri.2014.03.006 }}</ref><ref>{{cite journal | vauthors = Hornef M, Penders J | title = Does a prenatal bacterial microbiota exist? | language = En | journal = Mucosal Immunology | volume = 10 | issue = 3 | pages = 598–601 | date = May 2017 | pmid = 28120852 | doi = 10.1038/mi.2016.141 | doi-access = free }}</ref>