Estetrol is an estrogen steroid hormone exclusively produced by fetal liver from estradiol and estriol during pregnancy. It binds to estrogen receptors (ERs) with 4-fold higher affinity for ERα over ERβ and exhibits estrogenic effects in most tissues expressing ERs (bone, brain, vagina, and endometrium). In breast tumors, however, estetrol acts as an estrogen antagonist in the presence of estradiol, preventing tumor growth.
