Lactation describes the secretion of milk from the mammary glands and the period of time that a mother lactates to feed her young. The process naturally occurs with all sexually mature female mammals, although it may predate mammals. The process of feeding milk in all animals (including humans) is called nursing, and in humans it is also called breastfeeding. Newborn infants often produce some milk from their own breast tissue, known colloquially as witch's milk. In most species, lactation is a sign that the female has been pregnant at some point in her life, although in humans and goats, it can happen without pregnancy. Nearly every species of mammal has nipples; except for monotremes, egg-laying mammals, which instead release milk through ducts in the abdomen. In only one species of mammal, the Dayak fruit bat from Southeast Asia, is milk production a normal male function. Galactopoiesis is the maintenance of milk production. This stage requires prolactin. Oxytocin is critical for the milk let-down reflex in response to suckling. Galactorrhea is milk production unrelated to nursing. It can occur in males and females of many mammal species as result of hormonal imbalances such as hyperprolactinaemia. Purpose The chief function of a lactation is to provide nutrition and immune protection to the young after birth. Due to lactation, the mother-young pair can survive even if food is scarce or too hard for the young to attain, expanding the environmental conditions the species can withstand. The costly investment of energy and resources into milk is outweighed by the benefit to offspring survival. In almost all mammals, lactation induces a period of infertility (in humans, lactational amenorrhea), which serves to provide the optimal birth spacing for survival of the offspring. Human Hormonal influences From the eighteenth week of pregnancy (the second and third trimesters), a woman's body produces hormones that stimulate the growth of the milk duct system in the breasts: Progesterone influences the growth in size of alveoli and lobes; high levels of progesterone inhibit lactation before birth. Progesterone levels drop after birth; this triggers the onset of copious milk production. Estrogen stimulates the milk duct system to grow and differentiate. Like progesterone, high levels of estrogen also inhibit lactation. Estrogen levels also drop at delivery and remain low for the first several months of breastfeeding. Breastfeeding mothers should avoid estrogen-based birth control methods, as a spike in estrogen levels may reduce a