Lactation and the physiology of prolactin secretion.

Prolactin in man appears to form an essential part of the complex of hormones necessary for milk secretion and lactation. Levels of prolactin during pregnancy gradually increase towards term, and remain elevated for up to 6 weeks post partum. The lactogenic effects of prolactin appear to be blocked at the mammary gland by the elevated levels of fetoplacental steroids secreted during pregnancy. The immediate decline in steroid levels at delivery removes this block to prolactin and milk secretion ensues. The amenorrhoea associated with both puerperal lactation and galactorrhoea appears to reflect failure of cyclical discharge of gonadotrophins and anovulation. This appears to be due to peripheral inhibition of steroidogenesis by the elevated levels of prolactin associated with these situations. No consistent changes in circulating levels of prolactin occur during the menstrual cycle, but changes in prolactin levels within the follicular fluid of the developing ovarian follicle indicate a specific and permissive role of prolactin in steroidogenesis.