Placental expulsion

Placental expulsion (also called afterbirth) occurs when the placenta comes out of the birth canal after childbirth. The period from just after the baby is expelled until just after the placenta is expelled is called the third stage of labor.

The third stage of labor can be managed actively with several standard procedures, or it could be managed expectantly (also known as physiological management or passive management), the latter allowing the placenta to be expelled without medical assistance.

Although uncommon, in some cultures the placenta is kept and consumed by the mother over the weeks following the birth. This practice is termed placentophagy.

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.

As the HPA axis activates the posterior pituitary of the fetus begins to increase production of oxytocin, which stimulates the maternal myometrium to contract.

In the seventh month of pregnancy the MHC-I complexes increase in the interplacentomal arcade reducs the bi- and tri-nucleate cells, a source of immune suppression in pregnancy. By the ninth month the endometrial lining has thinned (due to loss of trophoblast giant cells) which exposes the endometrium directly to the fetal trophoblast epithelium. With this exposure and the increase in maternal MHC-I, T-helper 1 (Th1) cells, and macrophages induce apoptosis of trophoblast cells and endometrial epithelial cells, facilitating placental release. Th1 cells attact an influx of phagocytic leukocytes into the placentome at separation, allowing further degration of the extracellular matrix.

After delivery, loss of fetal blood return to the placenta allows for shrinkage and collapse of the cotyledonary villi with subsequent fetal membrane separation

Methods of active management include umbilical cord clamping, stimulation of uterine contraction and cord traction.

Active management routinely involves clamping of the umbilical cord, often within seconds or minutes of birth.

Uterine contraction assists in delivering the placenta, and can be induced with medication, usually via intramuscular injection. The use of ergometrine, on the other hand, is associated with nausea or vomiting and hypertension.

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