Milk secretion begins within a few hours of parturition. The initiating stimulus is the fall in circulating oestrogen and progesterone which occurs after delivery of the placenta, the main source of these hormones in pregnancy. Oestrogen and progesterone inhibit the lactogenic effects of the circulating anterior pituitary hormone prolactin, secretion of which gradually increases throughout pregnancy until the levels just before term are some 10–100 times non-pregnant values. With removal of the oestrogen and progesterone block, prolactin is free to initiate milk secretion.
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