Estrogen is any of several steroid hormones produced mainly by the ovaries and responsible for promoting estrus and the development and maintenance of female secondary sex characteristics. Estrogen is an essential part of a woman’s reproductive process. It regulates the menstrual cycle and prepares the uterus for pregnancy by enriching and thickening the endometrium. Estrogens are steroid hormones which readily diffuse across the cell membrane. Once inside the cell, they bind to and activate estrogen receptors which in turn up-regulate the expression of many genes. Additionally, estrogens have been shown to activate a G protein-coupled receptor, GPR30.
Estrogens are used in estrogen replacement therapy for postmenopausal women, and as part of some oral contraceptives. The three major naturally occurring estrogens in women are estrone (E1), estradiol (E2), and estriol (E3). Estradiol (E2) is the predominant form in nonpregnant females, estrone is produced during menopause, and estriol is the primary estrogen of pregnancy. In the body these are all produced from androgens through actions of enzymes.