Editing Placenta (section)

===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" />