Estrogens (also called oestrogens) are a group of steroid compounds that function as the primary female sex hormone. They are produced primarily by developing follicles in the ovaries, the corpus luteum and the placenta. Some estrogens are also produced in the adrenals and the breasts, especially in post-menopausal women.
The three naturally occurring estrogens are estradiol (estradiol), estriol (estriol) and estrone (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.
While estrogens are present in both men and women, it is found in women in significantly higher quantities. They promote the development of female secondary sexual characteristics, such as breasts, and are also involved in controlling the menstrual cycle, which is why most birth control pills contain estrogens.