A PARTURITION

Parturition is the end point of a succession of endocrine events involving maternal, fetal and placental interactions. The major hormones involved in the onset and maintenance of human parturition are oestrogens, progesterone, relaxin, oxytocin, prostaglandins, catecholamines, cortisol and β-endorphin. Oestrogens, relaxin and prostaglandins promote cervical ripening; prostaglandins, progesterone, oestrogens and oxytocin regulate myometrial activity. Catecholamines and cortisol help regulate the energetics of uterine contraction, and β-endorphin acts as a pain modulator. The release of β-endorphin (which is substantially reduced by epidural anaesthesia or by analgesics) is a response to the stress of labour and mirrors plasma cortisol levels; that is, plasma β-endorphin levels rise during labour, reach a peak at delivery, then fall to non-pregnant levels within 24–48 hours thereafter.

Pioneering work carried out in the 1970s by Liggins and his collaborators (National Women's Hospital, Auckland, New Zealand) provided evidence that in sheep the fetus plays a major role in initiating its own delivery. Subsequent research demonstrated that fetal adrenal cortisol triggers the cascade of maternal endocrine changes that, in turn, promote myometrial responsiveness to prostaglandins and oxytocin. However, the role of the fetal pituitary–adrenal axis in the onset of parturition varies from pivotal, as in the sheep, to uncertain in other species such as primates. In humans (and monkeys) exogenous glucocorticoids fail to induce parturition, suggesting that the mechanism determining gestation length and the onset of parturition is more complicated than in sheep. The length of pregnancy in a given species is determined by the fetal genotype and can be confined to a strict timing schedule, as in many seasonal breeders, or be more variable.