Release of prolactin is not an important component of initiation of parturition in humans. Prolactin promotes the growth of mammary alveoli, and aids in production of milk.
The signals for parturition originate from the fully developed foetus and the placenta which induce mild uterine contractions called foetal ejection reflex. This triggers release of oxytocin from the maternal pituitary. Oxytocin acts on the uterine muscle and causes stronger uterine contractions, which in turn stimulates further secretion of oxytocin. High levels of estrogens cause the number of receptors for oxytocin on uterine muscle fibers to increase. Estrogen also stimulates the placenta to release prostaglandins, which induce production of enzymes that digest collagen fibers in the cervix, causing it to soften. Progesterone inhibits uterine contraction, labor cannot take place until the effects of progesterone are diminished. Toward the end of gestation, the levels of estrogens in the mother’s blood rise sharply, producing changes that overcome the inhibiting effects of progesterone.