Oxytocin

Oxytocin is a hormone released from the pituitary gland in the brain. During pregnancy, this hormone causes labor contractions to start. It also is released when a woman's breasts are stimulated by suckling or pumping, causing milk to move from the ducts and out the tiny holes in the nipple. This is called the let-down reflex. In the first few days after delivery, oxytocin also causes uterine contractions. They help shrink the uterus back to the size it was before pregnancy.