The average woman has about 400 reproductive cycles during her lifetime. As time goes on, the reproductive organs begin to respond to lowering hormone levels, causing periods to become first irregular, and then stop altogether. Every woman does this at her own rate, but the progression is somewhat predictable.

Follicles and Hormones

In every cycle, the woman's pituitary gland produces follicle-stimulating hormones (FSH) that trigger the follicle cells (small pouches) in the ovary that surround the developing eggs to produce estrogen, which in turn prepares an egg (usually just one) for fertilization. Each month, 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 in turn 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 isn't fertilized, it, too, is reabsorbed and the lining of the uterus is shed during your period. The body's level of estrogen then dips, which triggers the FSH production that starts the whole cycle again.

“Running Out” of Eggs

Until recently it was thought that every woman was born with a set number of eggs, ranging from 400,000 to 700,000, and that half of those eggs deteriorated and were reabsorbed by each girl's body before she reaches puberty. Scientists are still researching whether this is accurate and how it works. Some new studies in mice suggest that there may be “germ cells” that develop into eggs throughout a woman's life. Whether these germ cells replenish your eggs or not, it does seem that over time 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. You may continue to have menstrual periods after you stop ovulating, since your body continues to produce some estrogen. Most women notice a change in the frequency, duration, and flow of their periods during the three to four years before they stop menstruating completely. That's why you can't truly know that you've gone through menopause until a full twelve months have passed since your last period.