Usually progesterone works on top of estrogen, and they have something in common for the development of female reproductive organs and breasts. In terms of breast development, estrogen promotes the development of milk ducts, darkening of the nipples and coloring of the areola. In addition, it also promotes the development of secondary female sexual characteristics, and progesterone promotes the development of breast follicles, which can prepare for breastfeeding in the future. In other aspects, estrogen promotes the proliferation and differentiation of cells in the basal layer of the vaginal epithelium, thickening the mucous membrane and increasing the glycogen content to maintain an acidic environment in the vagina. Progesterone accelerates the shedding of vaginal epithelial cells and acts on top of estrogen in the uterus and in the fallopian tubes.