So I was trying to figure out how exactly progesterone helps with breast development (some threads claim that it balances out estrogen, and others say it's just anti-estrogenic, it's always been confusing).
"It appears that estrogens and progestogens act through and with proto-oncogenes and growth factors to affect breast cell proliferation and breast cancer etiology. Animal studies suggest that estrogen causes interlobular ductal cell division and progesterone causes increased terminal duct lobular unit cell division in the luteal phase..........Their reproduction is also increased during the luteal phase. Yet, there is considerable interpersonal variation. No studies examining breast cell division have compared cell division rates with serum hormone concentrations, however. The peak of mitosis occurs about 3 days before breast cell death in the late luteal and very early follicular phases."
This article suggests that mitosis peaks in week 4 of the menstrual cycle (*4 compared to week 2), and both E/P induces cell proliferation. Progesterone is primarily responsible for the development of the terminal duct lobules (see attached image), and estrogen for the ducts connecting the lobules.