Estrogen

The immediate reason that women stop cycling is that their ovaries become depleted of eggs. Humans stop making eggs even before they are born and the seven million eggs contained in the ovaries of a five-month female fetus are all she will ever have. At birth, females have only one to two million eggs, and by puberty, only one-thirtieth of the original seven million remain. During each menstrual cycle, one egg becomes fully developed and will be ovulated. Somewhere between ages thirty-five and forty-five, the constant rate of egg loss suddenly accelerates about two-fold, so that at menopause only about one thousand eggs are left. It is believed that the alteration in the regularity of cycles compromises the feedback interaction between the ovaries and the neuroendocrine system, ultimately leading to a sharp decrease in the levels of estrogen and progesterone.

The dramatic reduction in the level of estrogen is associated with a wide variety of physiological changes that correlate with menopause. Indeed, as many as four hundred different actions of estrogen have been identified, affecting such diverse systems as circulation, brain activity, sexual behavior, bone biology, sleep patterns, intestinal absorption of food, and immune activity. Although menopause is clearly associated with such negative health effects as osteoporosis and heart disease, not all the alterations of menopause are detrimental. For example, before menopause, the risk of American women developing breast cancer doubles every three years, whereas after menopause it takes thirteen years for the risk to double again.