Although estrogen doesn’t cause cancer directly, it does set up the body for a pre-cancerous condition, and it does stimulate cancer that is already present. This effect was first seen decades ago when estrogen was given without an opposed progesterone compound;  the unopposed estrogen caused an 8-fold increase in cancer of the endometrium (the lining of the uterus).

The addition of progesterone is generally thought to eliminate the risk of endometrial cancer.  However, a few studies have indicated that long cycle HRT regimens, where  progesterone is only used every 3 months, may not offer complete protection.

Progesterone is added (or opposed) to estrogen to eliminate the risk of endometrial disease.  Women who have had a hysterectomy do not need opposed estrogen for this purpose. There is some debate whether a small amount of micronized natural progesterone might be useful in these women to  increase bone density.

Other risk factors for endometrial disease include:

Obesity