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Lactation describes the secretion of milk from the mammary glands, the process of providing that milk to the young, and the period of time that a mother lactates to feed her young. The process occurs in all female mammals, and in humans it is commonly referred to as breastfeeding. In most species milk comes out of the mother’s nipples; however, the platypus (a non-placental mammal) releases milk through ducts in its abdomen. Only one male mammal is known to produce milk, the Dayak fruit bat. Some other male mammals may produce milk as the result of a hormone imbalance, (for instance ‘witch’s milk’). This phenomenon may also be observed in newborn infants as well.


The chief function of lactation is to provide nutrition to the young after birth. In almost all mammals lactation, or more correctly the suckling stimulus, induces a period of infertility, usually by the suppression of ovulation, which serves to provide the optimal birth spacing for survival of the offspring. McNeilly, A. S. 1997. Lactation and fertility. Journal of Mammary Gland Biology and Neoplasia 2:291-298 PMID 10882312

Lactation as the result of Hormone Replacement Therapy

During Hormone Replacement Therapy it is possible due to fluctuations in hormone levels for some (but not all) patients to experience varying degrees of lactation. This is a possible normal side effect, and should not be worried about. Usually the lactation will stop as the body adjusts to new hormone levels. Excessive lactation could be a sign of higher than normal progesterone or Prolactin levels, and if it does not subside it would be wise to seek a hormone level test and consider if the levels are too high to have your hormone regime adjusted accordingly.

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