Lactation

Mammogenesis in pregnancy: estrogen drives ductal growth, progesterone lobuloalveolar development, aided by prolactin, hPL, cortisol and insulin. Lactogenesis I (from mid-pregnancy): alveolar cells begin to make milk components and small volumes of colostrum, but high progesterone blocks copious secretion. Lactogenesis II: after delivery of the placenta, progesterone falls while prolactin remains high, and the milk "comes in" ≈2–4 days postpartum. Galactopoiesis (maintenance) depends on regular milk removal: suckling triggers prolactin surges, and accumulated milk inhibits its own production locally (feedback inhibitor of lactation).

The milk-ejection (let-down) reflex: nipple stimulation → afferent nerves → hypothalamic paraventricular and supraoptic nuclei → oxytocin release → contraction of myoepithelial cells around alveoli and ducts within ≈30–60 s. The reflex can be conditioned (a baby's cry) and is inhibited by pain, stress and anxiety. Oxytocin also contracts the uterus (afterpains), aiding involution.

Composition: colostrum (first days) is rich in protein, secretory IgA, lactoferrin, leucocytes and growth factors; mature milk provides ≈65–70 kcal/100 mL with ≈7 g/dL lactose, ≈3.5–4 g/dL fat and ≈1 g/dL protein (whey-predominant), plus lipase, oligosaccharides that feed beneficial gut bacteria, and highly bioavailable iron but little vitamin K or D. Output at full lactation is ≈750–800 mL per day.