Estrogen is a group of compounds named for their importance in reproductive and menstrual cycles. They are the primary female sex hormones. Natural estrogen is a steroid hormone, while some synthetic ones are non-steroidal.
Estrogens are used as part of some oral contraceptives, in estrogen replacement therapy for postmenopausal women, and in hormone replacement therapy for trans women.
Estrogens are synthesized in all vertebrates as well as some insects. Their presence in both vertebrates and insects suggests that estrogenic sex hormones have an ancient evolutionary history.