Estrogen is a hormone that comprises a group of compounds, including estrone, estradiol and estriol. It is the main sex hormone in women and is essential to the menstrual cycle. Although estrogen exists in men as well as women, it is found in higher amounts in women, especially those capable of reproducing.

Estrogen contributes to the development of secondary sex characteristics, which are the defining differences between men and women that don’t relate to the reproductive system. In women, these characteristics include breasts, a widened pelvis, and increased amounts of body fat in the buttock, thigh and hip region. Estrogen also contributes to the fact that women have less facial hair and smoother skin then men.

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. Two hormones, the luteinizing hormone (LH) and the follicle stimulating hormone (FSH), help to control how the body produces estrogen in women who ovulate.

Estrogen is manufactured mostly in the ovaries, by developing egg follicles. In addition, estrogen is produced by the corpus luteum in the ovary, as well as by the placenta. The liver, breasts and adrenal glands may also contribute to estrogen production, although in smaller quantities.

Estrogen can be broken down into three distinct compounds: estrone, estradiol and estriol. During a woman’s reproductive life, which starts with the onset of menstruation and continues until menopause, the main type of estrogen produced is estradiol. Enzymatic actions produce estradiol from androgens. Testosterone contributes to the production of estradiol, while the estrogen estrone is made from andostenedione.

Estrogen is important to a woman’s health, not only as it relates to her reproductive cycle. Although it can cause women to retain fluid, and early exposure through early menses can increase a woman's risk of developing breast cancer, estrogen has its benefits. It can contribute to increasing high density lipoprotein (HDL), which is considered the “good” cholesterol, and lower the low density lipoprotein (LDL), which is the “bad” cholesterol.

After menopause, women experience a reduction in estrogen. This can lead to vaginal dryness, memory problems, hot flashes, fatigue, irritability and possibly one of the most devastating problems, a decrease in bone density. Although estrogen replacement therapy has been controversial, with medical professionals debating its safety and effectiveness, menopausal women should consult with a doctor for the safest and most effective way of dealing with the loss of estrogen.