Estriol is one of the three major naturally occurring estrogens, the others being estradiol and estrone.
Estriol is produced almost exclusively during pregnancy and is the major estrogen produced in the normal human fetus. During pregnancy the production of estriol depends on an intact maternal-placental-fetal unit. Fetal-placental production of estriol leads to a progressive rise in maternal circulating levels reaching a late-gestational peak several orders of magnitude greater than non-pregnant levels.
Estriol has been suggested to be less carcinogenic than estradiol and estrone in animal studies. It has been shown that at doses effective for the relief of postmenopausal symptoms, estriol does not induce endometrial proliferation to the extent of the other estrogens.