The mechanism by which labour is initiated is very poorly understood in humans. Oxytocin from the pituitary regulates the expulsive phase of labour and following delivery the contraction of the uterus to reduce blood loss. The corpus luteum produces Relaxin, a large polypeptide hormone which is able to prepare the cervix for delivery in many animals but thought to be of minor importance in humans. The levels of prostaglandins appear to be critical for human parturition since they soften and dilate the cervix and initiate uterine contractions. Indeed, prostaglandin inhibitors are administered to pregnant women to prevent preterm labour.
Lactation occurs after parturition when there is a sudden drop in oestrogen and progesterone levels.
Once milk is formed and stored in the mammary gland its release is dependent upon neural signals travelling to the hypothalamus via somatic nerves from the nipple and the spinal cord. The continued production of prolactin is dependent upon continued suckling by the infant which sends signals to the hypothalamus to inhibit the release of prolactin-release inhibiting hormone. The start of normal ovulatory cycles may be disrupted for several months following childbirth and this is probably due to the suckling stimulus inhibiting FSH and LH production. The production of milk can continue for several years after childbirth. In contrast to anterior pituitary function the posterior part of the pituitary receives direct neural connections from hypothalamic nuclei.