What is Estradiol (E3)? Estrogen is the main female sex hormone, and estriol (E3) is a form of estrogen. In contrast to estrone (E1) and estradiol (E2), estriol (E3) is secreted mainly by the placenta rather than the ovaries and is the dominant estrogen during pregnancy. Levels of estriol in women who are not pregnant … Continue reading Estriol (E3)