when does the uterine lining become more vascular
Endometrium The endometrium is the inner epithelial layer, along with its mucous membrane, of the mammalian uterus. It has a basal layer and a functional layer; the functional layer thickens and then is shed during menstruation in humans. Some other mammals including apes, Old World monkeys, some species of bat , and the elephant shrew menstruate.[1] In most other mammals the endometrium is reabsorbed in the estrous cycle. During pregnancy, the glands and blood vessels in the endometrium further increase in size and number. Vascular spaces fuse and become interconnected, forming the placenta, which supplies oxygen and nutrition to the embryo and fetus.[2][3] The speculated presence of an endometrial microbiota[4] has been argued against.[5][6]
Placental expulsion As the fetal hypothalmus matures activation of the HPA axis (hypothalmic-pituitary-adrenal axis) initiates labour though two hormonal mechanisms. The end pathway of both mechanisms leads to myometrial contractions a mechanical cause of placental separation, from the shear force, contractile and involution changes in the uterus distorting the placentome.