Estrogen is a hormone that plays a key role in the functioning of the menstrual cycle, fertilization, and pregnancy. Estrogen is also the main driver in the development of breasts, wider hips, and other secondary sex characteristics typically seen in those assigned female at birth.  Estrogen may also used by trans women and other trans-feminine people as part of their gender affirmation process.

The hormones commonly considered to be "sex hormones" in the body are testosterone, estrogen, and progesterone. Testosterone is often referred to as a "male" hormone, and estrogen and progesterone are often referred to as "female" hormones. However, it is interesting to note that no exclusively "male" or "female" hormones have been identified. All hormones characterized to date are present in all people regardless of sex, as are the receptor mechanisms that respond to those hormones. 

In fact, the observation of the sexes we call "male" and "female" in nature is in part the result of differences in the amounts of individual hormones in the body and differences in their patterns of secretion (first in utero and then again during puberty) rather than their strict presence or absence. In other words, testosterone, estrogen, and progesterone are produced by all people, but in differing amounts and in different patterns.

Like all hormones, estrogen is a chemical messenger secreted by one body tissue that travels to another tissue to affect growth and behavior. Hormones are secreted in bursts and in amounts that change hourly and daily, as well as varying during stages of life or phases of fertility.

In people assigned female at birth, estrogen is mostly produced in the ovaries. Follicle-stimulating hormone (FSH) stimulates the ovaries to produce estrogen. In all people, estrogen is also made by fat cells, the adrenal glands, and other tissues, but in much lower amounts.

Other than its role in reproduction and in the development of secondary sex characteristics, estrogen is also important for protein metabolism, water retention, and bone formation. Working with vitamin D and calcium, estrogen helps to break down and rebuild bone tissue. Estrogen levels decline after menopause. With less estrogen, the process of rebuilding bones slows, increasing the risk of osteoporosis. Decreasing estrogen levels can also lead to hot flashes, itching, vaginal dryness, and other conditions.