Menopause occurs naturally when a woman's ovaries run out of functioning eggs. At the time of birth, most females have about 1 to 3 million eggs, which are gradually lost throughout a woman's life. By the time of girls' first menstrual period, they have an average of about 400,000 eggs. By the time of menopause, a woman may have fewer than 10,000 eggs. A small percentage of these eggs are lost through normal ovulation (the monthly cycle). Most eggs die off through a process called atresia.
Normally, FSH, or follicle-stimulating hormone (a reproductive hormone), is the substance responsible for the growth of ovarian follicles (eggs) during the first half of a woman's menstrual cycle. As menopause approaches, the remaining eggs become more resistant to FSH, and the ovaries dramatically reduce their production of a hormone called estrogen.
Estrogen affects many parts of the body, including the blood vessels, heart, bone, breasts, uterus, urinary system, skin, and brain. Loss of estrogen is believed to be the cause of many of the symptoms associated with menopause. At the time of menopause, the ovaries also decrease their production of testosterone -- a hormone involved with sex drive.