Menopause is the cessation of ovulation and menstrual periods. The supply of follicles in the ovaries is depleted, increasing the amount of follicle-stimulating hormone (FSH), while decreasing the amount of estrogen and progesterone. The process may take one to two years. The years preceding the final menstrual period are known as perimenopause. Menopause usually occurs between ages 45 and 55; the average age in the United States is 51 to 52 years.