Release of Prolactin is not an important component of initiation of parturition in humans.
Towards the end of pregnancy, the increasing ratio of estrogen to progesterone promotes uterine contractions. Progesterone no longer inhibits them. High levels of estrogens increase the number oxytocin receptors and gap junctions in the myometrium. Prostaglandins induce the softening of uterine cervix and enhance uterine contractile strength.