The raw materials for making milk are transported by the mother’s blood to her breasts. The breasts transform these raw materials into the perfect food for the baby – milk. The composition of milk changes slightly as the baby’s needs change. Breastfeeding moms know the basic principle of breastfeeding: the balance between supply and demand, but its depths need to be continually revealed by scientific research. The breast is made up of secretory glands. Within these glands are grape-like strings of cells called alveoli. Milk is secreted from these vesicles. The secreted milk flows down the milk ducts to the opening in the nipple. The cellular vesicles and milk ducts form a lobe, and most women have 7-10 lobes of glandular tissue in each breast. In addition to these lobes that make milk, the breasts contain fat, nerves, blood vessels, and tissues that connect to each other. The size of the breast is determined only by the amount of fat tissue in it, and the amount of fat in the breast does not affect its ability to produce milk. Changes in the breasts occur early in pregnancy. The enlargement of the breasts is due to the estrogen and progesterone in the body stimulating the growth of cellular alveoli to prepare them for milk production. During pregnancy, the breasts have begun to make milk – colostrum. However, the production of large quantities of milk is dependent on the baby suckling after birth and hormonal changes. After the delivery of the placenta, the levels of estrogen and progesterone drop rapidly (they were higher during pregnancy) and are replaced by lactogen (the hormone that secretes milk through cellular vesicles). Milk production increases rapidly in the first few days after the baby is born, and the mom’s breasts swell up quickly “after milking”. When the baby begins to suckle on mom’s breast, his suckling stimulates nerves in the areola that send signals to mom’s brain. The brain releases another hormone, oxytocin. Oxytocin causes the cells around the alveoli to contract, squeezing the milk from the alveoli into the milk ducts, which in turn deliver it to the nipple. This is the very important “milk ejection reflex”, which effectively “empties” the breasts so that the baby can get enough milk. (The milk spray reflex is the scientific name for what we commonly call “milk bursts”. When a mother is tired, sick, or stressed, she may inhibit the milk ejection reflex, so that milk is secreted but not expelled effectively.) Continuous milk production depends on the baby removing milk from the breast on a regular basis. We have long known that one of the keys to successful breastfeeding is to allow the baby to breastfeed without restriction, frequently and effectively, especially in the first few weeks. But recent research has focused more on how a baby’s breastfeeding behavior affects a mother’s milk production, and how a mother’s ability to make and store milk affects how often her baby breastfeeds.