While giving birth reduces a woman's long-term risk of breast cancer, having a spontaneous or induced abortion does not 

a woman's risk of developing the disease, according to a reanalysis of data from 53 epidemiologic studies.

The studies included data on 83,000 women with breast cancer from 16 countries. Compared to women who have had no spontaneous or induced abortions, the researchers calculated that the overall relative risk of breast cancer for women who have had one or more spontaneous abortions was 0.98 (95% CI; 0.921.04); for women who have had one or more induced abortions, it was 0.93 (0.890.96, 

 Beral V, Bull D, Doll R, et al.; Collaborative Group on Hormonal   Factors in Breast Cancer. Breast cancer and abortion: collaborative reanalysis   of data from 53 epidemiologic studies, including 83,000 women with breast cancer   from 16 countries. 