Estrogen is any of a group of female sex hormones that stimulate the appearance of secondary female sex characteristics in girls at puberty. Estrogens control growth of the lining of the uterus during the first part of the menstrual cycle, cause changes in the breast during pregnancy, and regulate various metabolic processes. Among the better known estrogens are estrone, ethinyl estradiol, and estriol, all produced primarily in the ovaries. Stilbestrol and ethinyl estradiol, two synthetic estrogens, are respectively five and ten times as potent as estrone; their activity is similar to that of natural estrogens. They are used to treat various conditions, including estrogen deficiencies in women (most commonly after menopause) and inflammation of the vagina. They may be used to stimulate lactation following childbirth and in the treatment, but not cure, of advanced and even disseminated cancer of the prostate gland in men.