Lactation--the central control of reproduction.

We are used to thinking of lactation as a consequence of reproduction but we tend to forget that, in many marsupials and placental mammals, lactation serves the equally important function of spacing-out the interval between successive births. In primitive human communities, the duration of the birth interval was all-important for the survival of the young, and lactational amenorrhoea lasting up to two years was nature's most effective form of contraception. Today, particularly in urban areas, lactational amenorrhoea is short, even in those women who still breast-feed their babies, and this has been one of the major factors responsible for the recent rapid growth of the human population. There is still much speculation about the way in which lactation and nutritional status may act in concert to suppress reproductive activity. Afferent stimuli from the nipple are an essential component of the reflex and it is possible that, in addition to a generalized central inhibition, they may initiate secretory surges of prolactin that have a direct inhibitory action on the ovary itself. The evidence for these various mechanisms of action is discussed.