Lactation, secretion and yielding of milk by females after giving birth. The milk is produced by the mammary glands, which are contained within the breasts. The breasts, unlike most of the other organs, continue to increase in size after birth. Although mammary growth begins during pregnancy under the influence of ovarian and placental hormones, and some milk is formed, copious milk secretion sets in only after delivery. Since lactation ensues after a premature birth, it would appear that milk production is held back during pregnancy. The mechanism by which this inhibitory effect is brought about, or by which lactation is initiated at delivery, has long been the subject of an argument that revolves around the opposing actions of estrogen.

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