At approximately day 1

Estrogen Receptors α and β expressed in granulosa cells and oocytes.

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.

At approximately day 1

The epithelial cords become canalized, due to lysis of the central cells of the branches.

Duration: 4 weeks

Reference [PubMed Link]

Laurence DJ, Monaghan P, Gusterson, BA. 1991. The development of the normal human breast. Oxford Reviews of Reproductive Biology 13:149-174.