Estrogens (also oestrogens) are a group of steroid compounds that function as the primary female sex hormone. While estrogens are present in both men and women, they are found in women in significantly higher quantities. They promote the development of female secondary sexual characteristics, such as breasts, and are also involved in the thickening of the endometrium and other aspects of regulating the menstrual cycle, which is why many oral contraceptives contain estrogens. 

The three naturally occurring estrogens are estradiol, estriol and estrone. In the body these are all produced from androgens through enzyme action. Estradiol is produced from testosterone and estrone from androstenedione. Estrone is weaker than estradiol, and in post-menopausal women more estrone is present than estradiol. 

Why is estrogen prescribed
Estrogen is a hormone, a substance produced by the body. It is needed for normal growth and development of female sex organs and for functions such as bearing children. During menopause, the body's production of estrogen is decreased. Estrogen replacement is used for symptoms associated with menopause: hot flashes (feelings of warmth in the face, neck, and chest), sweating, sleep disturbance, vaginal discomfort (dryness and itchiness), poor concentration, and irritability. It also is used in the treatment of breast cancer in postmenopausal women and breast and prostate cancer in men. 

Estrogen is also used with diet, calcium supplements, and exercise to slow the progression of osteoporosis, a disease common in women after menopause resulting in bones that break easily.