Editing Placenta (section)

====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>