What hormone stimulates milk production?

Prolactin is the hormone that stimulates the mammary glands to produce milk.

Made primarily by lactotroph cells in the anterior pituitary gland, prolactin rises substantially during pregnancy. Estrogen and progesterone help enlarge and prepare the breasts, but their high concentrations during pregnancy also restrain full milk production. After delivery, those hormone levels fall sharply, allowing prolactin to promote lactation.

Nursing provides an important feedback loop. Suckling activates sensory nerves in the nipple, signaling the hypothalamus and pituitary to maintain prolactin release. Prolactin supports milk synthesis for subsequent feeds. It is different from oxytocin, which causes contraction of myoepithelial cells and produces the milk-ejection, or “let-down,” reflex during feeding.

The hormone has other roles as well, including effects on reproductive function and maternal behavior. Dopamine from the hypothalamus normally suppresses prolactin secretion, so medications that block dopamine receptors can raise prolactin levels. Conversely, dopamine-like drugs such as bromocriptine can lower it. This distinction explains why oxytocin is a tempting but incorrect answer: it helps release stored milk, while prolactin drives its production.