lactogenesis

Lactogenesis is the physiological process by which the mammary glands begin to secrete milk. It is commonly described as occurring in three overlapping stages: lactogenesis I, II, and III. The process is regulated by hormonal changes around childbirth and by breast stimulation from infant suckling, which also drives the let-down reflex via oxytocin. Lactation continues thereafter as galactopoiesis, the maintenance of milk production.

Lactogenesis I occurs from late pregnancy through the early postpartum period. During this stage, the mammary glands undergo secretory differentiation; alveolar cells synthesize components of milk, and small amounts of premilk or colostrum are produced. Colostrum is high in immunoglobulins and protein but low in fat and lactose.

Lactogenesis II begins with parturition and is characterized by rapid increase in milk production, known as secretory activation. The drop in progesterone after placental delivery relieves inhibition of prolactin, allowing gradual milk synthesis to intensify. Onset typically occurs 2-4 days after birth and can be accompanied by engorgement and breast fullness.

Lactogenesis III, or mature lactation, follows as milk composition and volume stabilize with ongoing suckling and demand-driven production.

Regulation: Hormonal control includes prolactin as primary milk synthase; oxytocin initiates milk ejection; estrogen and progesterone decline after birth; other metabolic hormones support production. Mechanical stimulation from nursing or pumping sustains prolactin release and milk supply. Several maternal factors may affect onset, such as diabetes, obesity, cesarean delivery, and breastfeeding technique.

Colostrum: In late pregnancy and the first days postpartum, colostrum precedes mature milk; it is rich in antibodies and immune factors but sparse in carbohydrate and fat. Mature milk develops over the first two weeks and comprises foremilk and hindmilk with varying fat and lactose content.