how does the placenta detaches from the uterus after birth
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.
Amniotic sac The amniotic sac, commonly called the bag of waters,[1] sometimes the membranes,[2] is the sac in which the fetus develops in amniotes. It is a thin but tough transparent pair of membranes that hold a developing embryo (and later fetus) until shortly before birth. The inner of these fetal membranes, the amnion, encloses the amniotic cavity, containing the amniotic fluid and the fetus. The outer membrane, the chorion, contains the amnion and is part of the placenta. On the outer side, the amniotic sac is connected to the yolk sac, the allantois and, via the umbilical cord, to the placenta.[3]