Young girls start menstruating between the ages of eleven and thirteen, when their reproductive systems reach maturity. Women have regular menstrual cycles every twenty-eight days until about the age of fifty, at which time menstruation becomes irregular. This irregularity signals the start of menopause. The natural cessation of menstruation occurs due to reduced production of the female hormones estrogen and progesterone, which generally occurs between the ages of forty and fifty-five. The age at which a woman enters menopause is affected by genetics, race, and environmental factors. Women can also go into premature menopause, either naturally or due to oophorectomy (the surgical removal of the ovaries).