No association exists between induced abortion and breast cancer in premenopausal women, according to the results of a prospective cohort study. And it's possible that an inverse association exists between spontaneous abortion, especially in very young women, and the disease.  

Researchers used the population of women that participated in the Nurses Health Study II. Of these women, approximately 16,000 or 15% reported having an induced abortion, and almost 22,000 or 21% reported having a spontaneous abortion.  

The hazard ratio for breast cancer among women who had one or more induced abortions was 1.01 (95% CI, 0.88–1.17) and for women having one or more spontaneous abortions was 0.89 (95% CI, 0.78–1.01). The relationship between each type of abortion and breast cancer was largely unchanged regardless of the number of abortions that occurred, age at the time of the abortion, parity, or timing of the abortion during the pregnancy.  