prolactin in lactation

Prolactin is the hormone that actually makes the milk. If oxytocin is the pump that pushes milk out, prolactin is the engine in the milk factory that keeps the product being manufactured in the first place.

Made by the anterior pituitary, prolactin rises steadily through pregnancy to prime the breast, but milk synthesis is held back until birth because high placental estrogen and progesterone block prolactin's action on the gland. Once the placenta is delivered and those hormones fall, prolactin can drive copious milk production. After that, each time the baby suckles, nerve signals from the nipple trigger surges of prolactin that keep milk being made.

Prolactin is unusual among pituitary hormones in being controlled mainly by inhibition: the hypothalamus continuously suppresses it with dopamine. Suckling reduces this dopamine brake, allowing prolactin to rise, which is why frequent feeding sustains supply.

A caveat: high prolactin outside of breastfeeding is not normal and can signal problems such as a pituitary tumor (prolactinoma) or medication effects, causing milk-like discharge, irregular periods or infertility. So prolactin is beneficial in its proper context but disruptive when chronically elevated otherwise.