Lactation is the physiological process by which the mammary glands produce, secrete, and expel milk after childbirth to provide complete nourishment to the newborn. It is an important physiological function of the female reproductive system and is regulated mainly by hormonal changes, breast stimulation, and maternal–infant interaction. The process of lactation begins during pregnancy, becomes established after delivery, and continues as long as milk removal from the breasts is maintained. The mammary glands undergo considerable structural and functional changes during pregnancy in preparation for lactation. Under the influence of oestrogen, progesterone, human placental lactogen, prolactin, cortisol, insulin, and other hormones, the breast tissue enlarges and the milk-producing alveoli and ducts develop. During pregnancy, prolactin levels increase markedly and stimulate the mammary alveolar cells to prepare for milk synthesis. However, the high concentrations of oestrogen and progesterone produced by the placenta inhibit the full secretion of milk during pregnancy. Thus, although the mammary glands are prepared for lactation, abundant milk production does not normally occur before delivery. After delivery, expulsion of the placenta causes a sudden fall in circulating oestrogen and progesterone levels. This removal of the inhibitory effect of these hormones allows prolactin to act effectively on the mammary alveolar cells. Within approximately two to three days after birth, there is a marked increase in milk secretion, commonly referred to as lactogenesis II or the onset of copious milk production. The physiology of lactation can be understood through three major processes: development of the mammary glands, synthesis and secretion of milk, and milk ejection or let-down. Prolactin is the principal hormone responsible for milk production. It is