With the expulsion of the placenta after a baby's birth, a drop in the hormones that maintained the pregnancy soon occurs and allows the hormone prolactin to begin to work. Prolactin tells the breasts it is time to begin producing large amounts of milk. A mother feels the result of prolactin when her milk comes in at around three to five days postpartum. Increased milk production usually occurs at this time even if a baby has not been breastfeeding well or often. However, frequent breastfeeding sometimes speeds up the process of establishing increased milk production. Occasionally, a mother experiences a delay in the production of large amounts of milk.

Effective breastfeeding requires effective sucking by the baby so that enough milk is transferred from the breast into the baby's mouth where it is swallowed. To suck effectively, a baby must latch deeply onto the breast and use the structures in his/her mouth to create intermittent (periodic) suction and also compress the milk ducts lying beneath the areola the area about 1-2 inches behind the nipple tip. Proper sucking signals the mother's body to release the hormone oxytocin, which results in a greater transfer of milk with the milk-ejection reflex (MER), or milk let-down.

If a baby is not breastfeeding effectively, increased milk removal or production can be accomplished through milk expression techniques. When using manual expression, a mother compresses the milk sinuses by hand to remove milk. Breast pumps remove milk by reducing resistance to milk flow out of the breast, and a few pumps have features that compress milk ducts to some degree.