In a Normal Mammalian Birth, How Does the Body Guarantee

In a normal mammalian birth, how does the body guarantee that birth proceeds to completion?

A) Via positive feedback -- stretching of the uterus by the descending fetus induces release of oxytocin from the posterior pituitary causing even stronger uterine contractions.
B) Via negative feedback -- stretching of the uterus by the descending fetus induces release of FSH and LH from the posterior pituitary causing even stronger uterine contractions.
C) Via positive feedback -- stretching of the uterus by the descending fetus induces release of prolactin from the posterior pituitary causing even stronger uterine contractions.
D) Via negative feedback -- stretching of the uterus by the descending fetus induces release of prostaglandins from the posterior pituitary causing weaker uterine contractions.