Editing Placenta (section)

====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.