Background:

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).[1],[2] In breast tumors, however, estetrol acts as an estrogen antagonist in the presence of estradiol, preventing tumor growth.[1],[2]

Reference:

[1]. Coelingh Bennink, H.J.T., Holinka, C.F., and Diczfalusy, E. Estetrol review: Profile and potential clinical applications Climacteric. 11(Suppl 1), 47-58 (2008).
[2]. Giretti, M.S., Guevara, M.M.M., Cecchi, E., et al. Effects of Estetrol on Migration and Invasion in T47-D Breast Cancer Cells through the Actin Cytoskeleton Front.Endocrinol.(Lausanne) 5(80), 1-8 (2014).