How Your Breasts Make Milk
Special cells inside your breasts make milk. These cells are called alveoli. When your breasts become fuller and tender during pregnancy, this is a sign that the alveoli are getting ready to work. Some women do not feel these changes in their breasts. Others may sense these changes after their baby is born.
The alveoli make milk in response to the hormone prolactin. Prolactin rises when the baby suckles. Another hormone, oxytocin causes small muscles around the cells to contract and move the milk through a series of small tubes called milk ducts. This moving of the milk is called the let-down reflex .
Oxytocin also causes the muscles of the uterus to contract during and after birth. This helps the uterus to get back to its original size. It also lessens any bleeding a woman may have after giving birth. The release of both prolactin and oxytocin may be responsible in part for a mother's intense feeling of needing to be with her baby
Another hormone, CCK (which stands for cholecystokinin), is released in both mother and baby during suckling, which causes both of you to feel sleepy. This is nature's way of helping you get additional rest, and helping your baby to be content until your milk volume increases.
Updated June 6, 2016