Oxytocin (from the Greek oxy, meaning “quick,” and tokos, meaning “childbirth”) stimulates contractions of the smooth muscle tissue in the wall of the uterus during childbirth. Prior to the late stages of pregnancy, the uterus is relatively insensitive to oxytocin. As the time of delivery approaches, the muscles become sensitive to increased secretion of oxytocin.

After delivery, oxytocin stimulates the ejection of milk from the mammary glands. The suckling of an infant stimulates the nerve cells in the brain (the hypothalamus) to release oxytocin. Once oxytocin is secreted into the circulatory system, special cells contract and release milk into collecting chambers from which the milk is released. This reflex is known as the milk let-down reflex.