At approximately day 1

Estrogen converting enzymes (17HSD1 and 17HSD7) expressed.

Reference [PubMed Link]

Vaskivuo TE, Maentausta M, Torn S, Oduwole O, Lonnberg A, Herva R, Isomaa V, Tapanainen JS. 2005. Estrogen receptors and estrogen-metabolizing enzymes in human ovaries during fetal development. Journal of Clinical Endocrinology & Metabolism 90(6):3752-3756.