At approximately day 1

Estrogen receptors first appear in the epithelium of the fetal breast.

Duration: 14 weeks

Reference [PubMed Link]

Howard BA, Gusterson BA. 2000. Human breast development. Journal of Mammary Gland Biology and Neoplasia 5(2):119-137.

At approximately day 1

Preantral follicles, which contain a large oocyte with a zona pellucida, multiple layers of granulosa cells, and a thecal cell layer (originating in the stroma) begin to develop at the border of the cortex and medulla.

Reference [PubMed Link]

Rabinovici J, Jaffe RB. 1990. Development and regulation of growth and differentiated function in human and subhumand primate fetal gonads. Endocrine Reviews 11(4):532-557.

At approximately day 1

Thecal cells (capable of steroidogenesis) are present.

Duration: 14 weeks

Reference [PubMed Link]

Rabinovici J, Jaffe RB. 1990. Development and regulation of growth and differentiated function in human and subhumand primate fetal gonads. Endocrine Reviews 11(4):532-557.