Rat Study

A 1980 study on rats showed that 77.7% of aborted rats developed breast cancer after being exposed to the carcinogen, DMBA. On the other hand, 66.7% of virgin rats and 0% of rats with full term pregnancies developed breast cancer after being exposed to the same carcinogen. The authors of the study concluded, "Therefore, while pregnancy and lactation (breastfeeding) protected the mammary gland (breast) from developing carcinomas (cancers) and benign lesions by induction of full differentiation (by inducing a maturing process), pregnancy interruption (abortion) did not elicit sufficient differentiation (maturity) in the gland to be protective." [Russo J, Russo IH. Susceptibility of the mammary gland to carcinogenesis. American Journal of Pathology 1980; 100:497-512] 

In a subsequent study, the authors wrote, "In women, protection against breast cancer is provided when pregnancy occurs before age 24. In contrast, abortion is associated with increased risk of breast cancer. The explanation for these epidemiologic findings is not known, but the parallelism between the DMBA-induced rat mammary carcinoma model and the human situation is striking." [Russo J, Tay TK, et al. Differentiation of the mammary gland and susceptibility to carcinogenesis. Breast Cancer Research and Treatment. 1982; 2:5-73]