Researchers from Brigham and Women's Hospital and Harvard Medical School in Boston studied over 100,000 nurses in the United States and found that women who had abortions and miscarriages were no more likely to have breast cancer than other women in the study. This prospective study began in 1993 with women 29 to 46 years of age who were cancer-free. During the 10-year study, 1,458 newly diagnosed cases of invasive breast cancer were reported. Between 1993 and 2003, 15% of participants reported having an induced abortion and 21% reported a history of spontaneous abortion. The hazard ratio for breast cancer among women who had one or more induced abortions was 1.01 (95% confidence interval, 0.88 to 1.17) after adjustment for established breast cancer risk factors. Among women with one or more spontaneous abortions, the covariate-adjusted hazard ratio was 0.89 (95% confidence interval, 0.78 to 1.01). The relation between induced abortion and the incidence of breast cancer did not differ materially by number of abortions, age at abortion, parity, or timing of abortion with respect to a full-term pregnancy.