Estrogen: A female steroid hormone that is produced by the ovaries and, in lesser amounts, by the adrenal cortex, placenta, and male testes. Estrogen helps control and guide sexual development, including the physical changes associated with puberty. It also influences the course of ovulation in the monthly menstrual cycle, lactation after pregnancy, aspects of mood, and the aging process. Production of estrogen changes naturally over the female lifespan, reaching adult levels with the onset of puberty (menarche) and decreasing in middle age until the onset of menopause. Estrogen deficiency can lead to lack of menstruation (amenorrhea), persistent difficulties associated with menopause (such as mood swings and vaginal dryness), and osteoporosis in older age. In cases of estrogen deficiency, natural and synthetic estrogen preparations may be prescribed. Estrogen is also a component of many oral contraceptives. An overabundance of estrogen in men causes development of female secondary sexual characteristics (feminization), such as enlargement of breast tissue.