Menopause is defined as the time when there have been no menstrual periods for 12 consecutive months and no other biological or physiological cause can be identified. It is the end of fertility, the end of the childbearing years. (A woman may still, however, be able to become pregnant unless 12 consecutive months have passed without a period.)
Induced menopause occurs if the ovaries are surgically removed (by bilateral oophorectomy) or damaged by radiation or drugs. Due to the abrupt cutoff of ovarian hormones, induced menopause causes the sudden onset of hot flashes and other menopause-related symptoms such as a dry vagina and a decline in sex drive. Early menopause (before age 40), whether natural or induced, carries a greater risk for heart disease and osteoporosis since there are more years spent beyond the protective cover of estrogen.
A "simple" hysterectomy (when the uterus but not the ovaries are removed) before natural menopause should not affect the production of sex hormones and so not cause menopause (unless the nerves or blood supply to the ovaries is damaged during the hysterectomy).
The timing of natural menopause is variable. In the western world, the average age at which menopause starts is now 51. Natural menopause can, however, be in a woman's 30s or 60s. Factors influencing the time of menopause include heredity (genetics) and cigarette smoking. Smokers (and former smokers) reach menopause an average of 2 years before women who have never smoked.
There is no relation between the time of a woman's first period and her age at menopause. The age at menopause is not influenced by a woman's race, height, number of children, or use of oral contraceptives.
The changes in the menopause transition (perimenopause) begin about 6 years before the natural menopause. This is a time when the levels of hormones produced by the aging ovaries fluctuate, leading to irregular menstrual patterns (irregularity in the length of the period, the time between periods, and the level of flow) and hot flashes (a sudden warm feeling with blushing). Other changes associated with perimenopause and menopause include night sweats, mood swings, vaginal dryness, fluctuations in sexual desire (libido), forgetfulness, trouble sleeping, and fatigue (probably from the loss of sleep).