Risk of breast cancer among white women following induced abortion.

Some studies (but not all) have suggested that there may be an increase in the risk of breast cancer associated with a prior induced abortion. The risk, if present, may vary according to the duration of the pregnancy in which the abortion occurred, or to a woman's age or parity at that time. The authors conducted a case-control study of breast cancer in white women under age 45 years to address the question of breast cancer risk in relation to induced abortion, with the intention of identifying subgroups of women who might be at particularly increased risk. White women who were diagnosed with breast cancer (n = 1,302) from May 1, 1990, through December 31, 1992, in three geographic regions of the United States (Atlanta, Georgia; Seattle/Puget Sound, Washington; and five counties in central New Jersey) were interviewed about their reproductive histories, including the occurrence of induced abortion. Similar information was obtained from control women identified through random digit dialing. Logistic regression analysis was used to estimate the relative risk of breast cancer associated with a history of induced abortion, controlling for the potentially confounding influence of other breast cancer risk factors. Among women who had been pregnant at least once, the risk of breast cancer in those with a prior induced abortion was 20% higher than that in women with no history of abortion (95% confidence interval 1.0-1.5). This small increase in risk varied little according to number of abortions or a woman's current age. The association was present primarily among nulliparous women whose abortions occurred prior to 9 weeks' gestation (estimated relative risk = 2.0, 95% confidence interval 1.2-3.3). There was no excess risk of breast cancer associated with induced abortion among parous women. These data support the hypothesis that there may be a small increase in the risk of breast cancer related to a history of induced abortion among young women of reproductive age. However, the data from this study and others do not permit a causal interpretation at this time; neither do the collective results of the studies suggest that there is a subgroup of women in whom the relative risk associated with induced abortion is unusually high.