The authors evaluated whether an induced or
spontaneous abortion during the first six months of gestation, particularly if it occurs before the first term pregnancy, increases the risk of
breast cancer. Data from a case-control study of women under 70 years of age were used: 3,200 cases of
breast cancer were compared with 4,844 controls with nonmalignant nongynecologic conditions. Among both nulliparous and parous women, the risk of
breast cancer was not related to the number of induced or
spontaneous abortions. After allowance for all identified potential confounding factors, the estimated relative risk for nulliparous women with an
induced abortion relative to those who had never been pregnant was 1.3 (95% confidence interval (CI) 0.8-2.2), and for
spontaneous abortion, the corresponding estimate was 0.9 (95% CI 0.5-1.5). Among parous women, the estimated relative risks were 1.2 (95% CI 0.9-1.6) for an
induced abortion and 0.9 (95% CI 0.8-1.0) for a
spontaneous abortion, relative to never having had an abortion of any type. The time of the abortion had little effect: The relative risk estimates were 0.9 (95% CI 0.5-1.4) for
induced abortion before the first term birth, 1.4 (95% CI 1.0-1.9) for
induced abortion first occurring after the first term birth, 0.9 (95% CI 0.7-1.2) for
spontaneous abortion before the first term birth, and 0.9 (95% CI 0.7-1.0) for
spontaneous abortion first occurring after the first term birth. Similar results were evident for women under age 40, among whom the frequency of
induced abortion was relatively high. These data suggest that the risk of
breast cancer is not materially affected by abortion, regardless of whether it occurs before or after the first term birth.