Such correlations strongly suggest that estrogen, especially if unopposed by progesterone, is somehow related to the development of breast cancer. Estrogen's job in the uterus is to cause proliferation of the cells. Under the influence of estrogen, uterine cells multiply faster, and then progesterone normally should come on the scene with ovulation and stop the cells from multiplying. Progesterone causes the cells to mature and enter into a secretory phase that causes the maturing of the uterine lining, which is now ready to receive a possible fertilized egg. Estrogen is the hormone that stimulates cell proliferation, or the growing phase.
Progesterone is the hormone that stops growth and stimulates ripening.
We have an epidemic of breast cancer and cancer of the uterus in the United States. We have evidence that breast cancer occurs most often at the stage in your life when estrogen is dominant for the full month and progesterone is not coming in at the halfway point at ovulation time. Dr. Graham Colditz of Harvard maintains that unopposed estrogen is responsible for 30 to 35 percent of the breast cancers. I would put that number even higher. The above information was taken from What Your Doctor May Tell You About Menopause , by Dr. John Lee.