Menopause is a natural biological part of women's aging, not a disease marked by estrogen deficiency. The generally accepted medical definition of natural menopause puts it at a point in time twelve months after a woman's final menstrual period. Thus menopause marks the end of her natural reproductive life; her ovaries have naturally slowed their estrogen production to a level at which ovulation and conception are no longer possible. Notice that menopause can only be identified retrospectively, after one year without menstruation; there is no independent biological marker of menopause.
The average age of menopause in the western world is about 51, but it can and does occur anywhere between about 42 and 58 years of age.
Menopause can also be induced by medical procedures. Women who undergo hysterectomy - a very common procedure in western countries - are said to have had surgical menopause. Chemotherapy or radiation therapy performed to treat conditions such as ovarian cancer will also induce menopause. Induced menopause can occur at any time during a woman's reproductive years.
Although menopause itself is a point in time, the entire process of "reproductive aging" is understood to take place over 15 years or so. First is premenopause, then perimenopause or climacteric (which may last for up to five years, till menopause itself), then the menopausal transition (basically the last period and the following 12 months), then the Big Event, and finally postmenopause, which continues until death. As the population ages, women are living a third of their lives in their postmenopausal years, a dramatic change from the situation in the past.
Menopause may occur naturally and easily, or it may be marked by symptoms so severe that they require medical treatment. The most common complaints are hot flashes, which are sudden and rather intense waves of heat and sweating that can disrupt sleep. Menstrual periods become irregular, and women may find themselves more suseptible to vaginal infections. They may experience dryness of the vagina which can make intercourse painful. Incontinence joins this happy list, along with mood swings, weight gain, and lack of interest in sex. About 75% of women in the west report symptoms of varying severity, but women in other cultures often have no symptoms at all.
Such experiential differences are not surprising: we live in sociocultural contexts which affect how we experience our bodies. Menopause in the west is associated with aging and loss of youth; women in cultures which respect age often experience menopause as a positive rather than a negative transition.
In addition, in the west menopause has been medicalized, as have so many other aspects of women's health, and so this natural process has been viewed as a disease which must be treated. One of the more popular treatments is the badly named hormone replacement therapy; a large randomized control trial has recently shown that this therapy increases the risk of breast cancer and heart disease and thus can cause substantial harm to some women. Some women find relief throught naturopathic treatments such as dong quai, homeopathy, soy products, and acupuncture, though most western-trained doctors will hasten to tell you that the effects are not proven. But neither was HRT, and I suspect the side effects tofu consumption will be a lot more benign.
Although there has been much talk of male menopause, the naturally occuring process whereby testosterone levels drop should be referred to andropause, not menopause. Menopause is related to the word menstruation.
www.menopause.org/aboutmeno/sga.pdf on how menopause is defined
www.ldb.org/menopaus.htm on cultural differences in the experience of menopause
www.mayoclinic.com/invoke.cfm?id=MC00003 on andropause