The average woman has about 400 reproductive cycles during her lifetime. In every cycle, the woman's pituitary gland produces follicle stimulating hormones (FSH) that trigger the follicle cells in the ovary that surround the developing eggs to produce estrogen, which in turn prepares an egg (usually just one) for fertilization. As the body's level of estrogen increases, the pituitary gland stops producing FSH and starts producing luteinizing hormone (LH), which causes the ovary to ovulate (release the egg) and produce progesterone, which prepares the uterine lining to accept the fertilized egg.
The mature egg is only one of several "candidates" available each month. Those that don't mature (develop enough to be available for fertilization) are reabsorbed by the body. If the mature egg is unfertilized, it, too, is reabsorbed and the lining of the uterus is shed in the normal menstrual flow. The body's level of estrogen dips, which then triggers the FSH production that starts the whole cycle again.
Every woman is born with a set number of eggs, ranging from 400,000 to 700,000. Half of those eggs deteriorate and are reabsorbed by each girl's body before she reaches puberty. Scientists are still researching why this occurs. With each month's ovulation, more of the egg supply is depleted. As you near menopause, your egg supply diminishes, your follicle cells stop responding to FSH, and you stop ovulating. As a result—over a period of years—you stop menstruating and your ovaries stop making estrogen and progesterone.
What Menopause Means for Your Hormones
Your body produces dozens of hormones, but only three of them play a major role in your reproductive cycle. Those three are estrogen, progesterone, and small quantities of androgens (testosterone, for example). Here's what those hormones do:
Estrogen is a growth hormone that stimulates the development of adult sex organs during puberty; helps retain calcium in bones; regulates the balance of "good" and "bad" cholesterol in the bloodstream; and aids other body functions, such as blood sugar level, memory functions, and emotional balance.
Progesterone balances the effects of estrogen by aiding the maturation of body tissues and limiting their growth; stimulates the uterus, breasts, and fallopian tubes to secrete nutrients necessary for the body to prepare for growing an embryo and bearing a child; and raises body temperature and blood sugar levels.
As you approach menopause, the amount of hormones your body makes is in continual fluctuation, leading to the time before menopause known as perimenopause.
