Induced abortion and breast cancer risk

Results from case-control studies suggest that induced abortion may be asso ciated with a small increase in risk of breast cancer. While risk estimates from cohort studies have generally not observed such an association, these studies have had limited information regarding abortion and possible confo unding variables. Therefore, we conducted a study among a cohort of post-me nopausal women from whom detailed information regarding pregnancy outcomes as well as risk factors for breast cancer had been collected. The study sam ple included 37,247 Iowa Women's Health Study participants, 55-64 years of age at baseline in 1986, who reported no history of breast, or other, cance r (except non melanoma skin cancer), and for whom information regarding pre gnancy outcomes (that is, live birth, stillbirth, spontaneous abortion, ect opic pregnancy or induced abortion) was available. We used linkage with rec ords of the State Health Registry of Iowa, part of the National Cancer Inst itute's Surveillance, Epidemiology, and End Results (SEER) program, to esti mate the incidence of breast cancer among cohort members through 1995. We c alculated age-adjusted relative risks and 95% confidence intervals using Co x proportional hazards regression. Only 653 women (1.8%) reported an induce d abortion. The age-adjusted relative risk of breast cancer among women wit h prior induced abortion compared with those without was 1.1 (95% CI = 0.8- 1.6). Relative risks were higher among women whose age at first abortion wa s less than 20 or at least 30 years, for those whose abortion took place af ter their first birth or who never gave birth, and for those with early ter mination (0-2 months). These estimates varied from 1.3-1.7, but the confide nce intervals around each were wide. Since most women in this cohere were b eyond their reproductive years when abortion became legal in 1973, the low prevalence of induced abortion argues for a cautious interpretation.