Estrogen is the primary sex hormone in women and it functions during the reproductive menstrual cycle. Women have three major types of estrogen: estrone, estradiol, and estriol, which bind to and activate receptors within the body. Researchers discovered the three types of estrogen over a period of seven years, contributing to more detailed descriptions of the menstrual cycle. Each type of estrogen molecule contains a slightly different arrangement or number of atoms that in turn causes some of the estrogens to be more active than others. The different types of estrogen peak and wane throughout women's reproductive cycles, from normal menstruation to pregnancy to the cessation of menstruation (menopause). As scientists better explained the effects of estrogens, they used that information to develop oral contraceptives to control pregnancy, to map the menstrual cycle, and to create hormone therapies to regulate abnormal levels of estrogen.

In the early 1920s, researchers Edgar Allen and Edward Adelbert Doisy conducted an experiment that demonstrated that ovarian follicles, which produce eggs in mammals, also contain and produce what they called the primary ovarian hormone, later renamed estrogen. In their experiment, Doisy and Allen extracted estrogen from the ovarian follicles of hogs and proved that they had isolated estrogen by using a measurement later renamed the Allen-Doisy test. Allen and Doisy’s 1923 experiment to isolate estrogen showed it was made within the ovaries and also established a method for isolating the sex hormone. That method provided a basis for future research on hormones. Later researchers showed that estrogen functions in the menstrual cycles of primates by signaling for the tissue lining the uterus (endometrium) to thicken in preparation for possible implantation of a fertilized egg.

Hormone replacement therapy, or HRT, is a form of medication often used to treat the symptoms of menopause. According to the National Institute on Aging, menopause is the point in a female’s life twelve months after she has had her last period. The time leading up to menopause, often called perimenopause, is a transition stage when levels of sex hormones, namely estrogen and progesterone, begin to fall. For approximately eighty-five percent of menopausal females, that decline results in symptoms such as vaginal dryness, shifting moods, and hot flashes, or an abrupt feeling of warmth typically in the upper body. HRT replenishes a person’s sex hormones, and though there are many methods of HRT, people most commonly take a pill that contains either estrogen only or both estrogen and progesterone. HRT has evolved scientifically but has at times resulted in controversy over potential side effects. Despite historical controversy, as of 2024, researchers recognize that with careful consideration of an individual's health conditions and history, HRT can be an effective treatment for menopausal symptoms.