1. the formation and release of milk by the mammary glands. It is a form of nursing that occurs only in female mammals and is divided into lactogenesis—the initiation of milk production—and galactopoiesis—the subsequent maintenance of milk production and secretion. The former is itself divided into Lactogenesis Stage I, which in humans generally lasts from mid-pregnancy through 2 days following childbirth, and Lactogenesis Stage II, which occurs within 3 to 8 days after birth. In Lactogenesis Stage I, the various mammary cells enlarge, reorganize into large clumps resembling grapes, and start producing a nutrient- and antibody-rich fluid known as colostrum that is eventually released during the initial days of nursing. In Lactogenesis Stage II, the enlarged mammary cells fill with copious amounts of mature milk once the maternal placenta is expelled and no longer introducing progesterone, which inhibits milk formation, into the bloodstream.

In the subsequent galactopoiesis phase, beginning around 9 days after birth, the mammary cells continuously produce mature milk in response to its repeated removal by the suckling infant or by a breast pump. Lactation is regulated primarily by the hormones prolactin, which facilitates and sustains milk production, and oxytocin, which stimulates the milk letdown reflex to faciliate milk excretion. Lactation is an energy-intensive process that places significant metabolic demands on the female system, requiring not only increased caloric intake but also considerable amounts of calcium and other minerals. See also galactorrhea.

2. the period during which this process occurs.