Lactation Physiology

Expansion

Prolactin (anterior pituitary)

• Stimulates milk synthesis
• Held in check during pregnancy by high progesterone; delivery of the placenta removes this brake, and lactation begins
• Maintained by suckling, which inhibits hypothalamic dopamine
• Also suppresses GnRH, causing lactational amenorrhoea, a natural contraceptive effect that is unreliable unless feeding is exclusive and frequent

Oxytocin (posterior pituitary)

• Causes milk ejection by contracting myoepithelial cells: the let-down reflex
• Can be conditioned, so the sight or cry of the infant triggers it, and it is inhibited by stress, pain and anxiety
• Also causes uterine contraction, which is why breastfeeding aids involution and causes afterpains

Colostrum is produced for the first few days: low in fat and lactose, high in protein and rich in secretory IgA, lymphocytes and lactoferrin, providing passive mucosal immunity.

Milk production is ultimately demand led, since retained milk suppresses further synthesis locally.