Estrogen is a group of hormones that play an important role in
the normal sexual and reproductive development in women. They
are also called sex hormones. The woman's ovaries produce most
estrogen hormones, although the adrenal glands also produce
small amounts of the hormones. In addition to regulating the
menstrual cycle, estrogen affects the reproductive tract, the
urinary tract, the heart and blood vessels, bones, breasts,
skin, hair, mucous membranes, pelvic muscles, and the brain.
Secondary sexual characteristics, such as pubic and armpit hair,
also begin to grow when estrogen levels rise. Many organ
systems, including the musculoskeletal and cardiovascular
systems, and the brain are affected by estrogen. To learn more
about women's health, and specifically hormone replacement
therapy, the National Heart, Lung, and Blood Institute of the
National Institutes of Health (NIH) launched the Women's Health
Initiative (WHI) in 1991. The hormone trial had two studies: the
estrogen-plus-progestin (HRT) study of