Twelve months without a menses (i.e., period) is called menopause. The average age at menopause is 51.4 years. Natural menopause is the result of ovarian aging. At 20 weeks gestation, there are 5-7 million ovarian follicles. At birth, 1.5-2 million follicles are present and at the onset of menses only 300,000-400,000 follicles remain. Females continue to deplete their follicles throughout life. There is an accelerated loss of follicles when their total number is less than 25,000. When the follicles are less than 1,000, menopause occurs.