Estriol (E3), also spelled oestriol, is a steroid, a weak estrogen, and a minor female sex hormone.[1][2] It is one of three major endogenous estrogens, the others being estradiol and estrone.[1] Levels of estriol in women who are not pregnant are almost undetectable.[3] However, during pregnancy, estriol is synthesized in very high quantities by the placenta and is the most produced estrogen in the body by far,[3][4] although circulating levels of estriol are similar to those of other estrogens due to a relatively high rate of metabolism and excretion.[4][5] Relative to estradiol, both estriol and estrone have far weaker activity as estrogens