Which hormone stimulates milk production?

• A. Estrogen
• B. Prolactin
• C. Progesterone
• D. Oxytocin

Answer: (B)

Milk production is driven by prolactin, a hormone from the anterior pituitary that signals the mammary alveolar cells to synthesize and secrete milk when the breast is stimulated by a nursing infant. The release of prolactin is normally increased when the baby suckles because nerve signals from the nipple reduce dopamine from the hypothalamus, lifting inhibition on prolactin and allowing it to rise.

Prolactin’s action is to stimulate the production of milk components—lactose, fats, and proteins—in the mammary glands. This production continues as long as there is ongoing nipple stimulation and sufficient prolactin activity. In contrast, oxytocin regulates milk ejection from the breast (the let-down reflex) but does not drive milk synthesis.

During pregnancy, estrogen and progesterone help develop breast tissue but actually suppress milk production until after birth. When the placenta is expelled, these hormones fall, and prolactin can act to produce milk in response to nursing.