Estriol (Estriol, oestriol, E3) is one of the three main estrogens in human. Metabolites of estradiol and estrone. Among estrone, estradiol and estriol, estriol has the weakest activity. It is present in urine, and its content is higher during pregnancy. There is no significant difference in serum estriol concentration among non-pregnant women, postmenopausal women and men.