Recent findings suggest that both the production of progesterone during pregnancy and the progesterone endogenously produced or exogenously administered outside pregnancy, does not increase breast cancer risk, and could even be protective.
Campagnoli C, Abba C, Ambroggio S, Peris C. (October 2005). Pregnancy, progesterone and progestins in relation to breast cancer risk. J Steroid Biochem Mol Biol. 2005 Oct 21; [E