Estrogen is produced in the egg follicle and interstitial cells of the ovaries, according to the Encyclopædia Britannica. The placenta is another major source of estrogen, with adrenal glands and male testes secreting small amounts of estrogen as well.
The term estrogen actually describes any one of the group of hormones that have a major influence on the function and development of the female reproductive tract. The three major hormones in the group are estradiol, estrone and estriol. Estradiol is the chief hormone in the group. Estrogen levels in the bloodstream tend to be at their highest during ovulation and menstruation.