* Expulsion of the foetus requires that the chorionic villi become dislodged from the crypts of the maternal side of the [[Female_Reproductive_Tract_-Placenta_-_Anatomy_%26_Physiology|placenta]].
−
** Brought about by powerful vasoconstriction of arteries in the villi.
−
*** Vasoconstriction reduces pressure and allows villi to be released from crypts.