Breast milk production, or lactation, is driven by hormones and the body's supply-and-demand system. Here's how it works:Hormonal Changes: During pregnancy, estrogen, progesterone, and prolactin prepare the breasts. Estrogen and progesterone grow milk ducts and alveoli (milk-making glands), while prolactin readies them for production.
Post-Birth: After delivery, progesterone drops, triggering milk production. Prolactin stimulates alveoli to make milk, and oxytocin causes the "let-down" reflex, releasing milk into ducts when the baby suckles or a pump is used.
Milk Creation: Alveoli cells pull nutrients, water, fats, proteins, and lactose from the blood to form milk:Colostrum: Thick, antibody-rich first milk for newborns (days 1–5).
Transitional Milk: A mix of colostrum and mature milk (days 3–14).
Mature Milk: Nutrient-balanced, with watery foremilk and fatty hindmilk.

Supply and Demand: Milk production depends on removal. Frequent breastfeeding or pumping boosts supply via prolactin, while less demand reduces it. A protein called feedback inhibitor of lactation (FIL) slows production if milk builds up.
Ongoing Adaptation: Regular feeding or pumping sustains milk production. Milk composition adjusts to the baby’s needs, influenced by age, feeding frequency, and time of day.

In short, breast milk is made through hormonal signals (prolactin, oxytocin), suckling or pumping, and nutrient extraction from the blood, adapting to the baby’s demands.