This process begins about 3 to 5 years before the final menstrual period. This transitional phase is called the climacteric, or perimenopause. Menopause is considered complete when a woman has been without periods for 1 year. On average, this occurs at about age 50. But like the beginning of menstruation in adolescence, timing varies from person to person.
Cigarette smokers tend to reach menopause earlier than nonsmokers.
The ovaries contain structures called follicles that hold the egg cells. A baby girl is 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.