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.
PIP: No evidence was found that a 1st trimester induced or spontaneous abortion before the 1st term pregnancy increased the risk of breast cancer. 3200 cases of breast cancer and 4844 controls were examined. For 820 nulliparous cancer patients and 1688 multiparous controls there was no evidence that the risk of breast cancer increased with the number of either induced or spontaneous abortions. The results were the same for women under 40 as for women over 40. For 2380 parous cancer patients and 3156 controls, none of the relative risk estimates were greater than 1.0, for either women over 40 or under 40. Risk estimates were calculated for both induced and spontaneous abortions both before and after a 1st term birth. None of the risks were significantly greater than 1.0, either for women under 40 or those over 40. The only conditions under which the risk of breast cancer related to abortion was significantly greater than 1.0 were in women whose 1st induced abortion occurred after the 1st term birth among premenopausal women (1.6) and among women with an early age at menarche (2.7). It is concluded that there is no evidence linking abortion to the risk of breast cancer.