How Does It Happen?
The ovaries contain structures called follicles that hold the egg cells. You are born with about 2 million egg cells and by puberty there are about 300,000 left. Only about 400 to 500 ever mature fully to be released during the menstrual cycle.

The rest degenerate over the years. During the reproductive years, the pituitary gland in the brain generates hormones that cause a new egg to be released from its follicle each month. The follicle also increases production of the sex hormones estrogen and progesterone, which thicken the lining of the uterus. This enriched lining is prepared to receive and nourish a fertilized egg following conception. If fertilization does not occur, estrogen and progesterone levels drop, the lining of the uterus breaks down, and menstruation occurs.

For unknown reasons, the ovaries begin to decline in hormone production during the mid-thirties. In the late forties, the process accelerates and hormones fluctuate more, causing irregular menstrual cycles and unpredictable episodes of heavy bleeding. By the early to mid-fifties, periods finally end altogether. However, estrogen production does not completely stop. The ovaries decrease their output significantly, but still may produce a small amount. Also, another form of estrogen is produced in fat tissue with help from the adrenal glands (near the kidney). Although this form of estrogen is weaker than that produced by the ovaries, it increases with age and with the amount of fat tissue.

Progesterone, the other female hormone, works during the second half of the menstrual cycle to create a lining in the uterus as a viable home for an egg, and to shed the lining if the egg is not fertilized. If you skip a period, your body may not be making enough progesterone to break down the uterine lining. However, your estrogen levels may remain high even though you are not menstruating.

At menopause, hormone levels don't always decline uniformly. They alternately rise and fall again. Changing ovarian hormone levels affect the other glands in the body, which together make up the endocrine system. The endocrine system controls growth, metabolism and reproduction. This system must constantly readjust itself to work effectively. Ovarian hormones also affect all other tissues, including the breasts, vagina, bones, blood vessels, gastrointestinal tract, urinary tract, and skin.

Surgical Menopause
Premenopausal women who have both their ovaries removed surgically experience an abrupt menopause. They may be hit harder by menopausal symptoms than are those who experience it naturally. Their hot flashes may be more severe, more frequent, and last longer. They may have a greater risk of heart disease and osteoporosis, and may be more likely to become depressed. The reasons for this are unknown. When only one ovary is removed, menopause usually occurs naturally. When the uterus is removed (hysterectomy) and the ovaries remain, menstrual periods stop but other menopausal symptoms (if any) usually occur at the same age that they would naturally. However, some women who have a hysterectomy may experience menopausal symptoms at a younger age.

What To Expect
Menopause is an individualized experience. Some women notice little difference in their bodies or moods, while others find the change extremely bothersome and disruptive. Estrogen and progesterone affect virtually all tissues in the body, but everyone is influenced by them differently.

Hot Flashes
Hot flashes, or flushes, are the most common symptom of menopause, affecting more than 60 percent of menopausal women in the U.S. A hot flash is a sudden sensation of intense heat in the upper part or all of the body. The face and neck may become flushed, with red blotches appearing on the chest, back, and arms. This is often followed by profuse sweating and then cold shivering as body temperature readjusts. A hot flash can last a few moments or 30 minutes or longer.

Hot flashes occur sporadically and often start several years before other signs of menopause. They gradually decline in frequency and intensity as you age. Eighty percent of all women with hot flashes have them for 2 years or less, while a small percentage have them for more than 5 years. Hot flashes can happen at any time. They can be as mild as a light blush, or severe enough to wake you from a deep sleep. Some women even develop insomnia. Others have experienced that caffeine, alcohol, hot drinks, spicy foods, and stressful or frightening events can sometimes trigger a hot flash. However, avoiding these triggers will not necessarily prevent all episodes.

Hot flashes appear to be a direct result of decreasing estrogen levels. In response to falling estrogen levels, your glands release higher amounts of other hormones that affect the brain's thermostat, causing body temperatures to fluctuate. Hormone therapy relieves the discomfort of hot flashes in most cases.

National Institute of Health Menopause

North American Menopause Society