Progesterone vs. Progestin
A progestin is a hormone that's very similar to the progesterone your body makes. The Women's Health Initiative study observed increased risks for breast cancer in women taking a progestin called medroxyprogesterone. Women who took both horse estrogens and medroxyprogesterone in the study had slightly higher risk for breast cancer.
Oddly, women in the Women's Health Initiative study who took only the horse estrogens, without medroxyprogesterone, did not have an increased risk for breast cancer.
Several studies have looked at women who have taken estradiol and progesterone, the hormones that are chemically identical to the estrogen and progesterone made by human females. In study after study, progesterone is found to be protective against breast cancer.
Women in France who took progesterone were shown in a study called "E3N Cohort" to have less breast cancer than women who took no hormones at all.