Neither induced abortion nor miscarriage was associated with breast cancer risk in this prospective cohort of female California teachers and administrators. This result is consistent with the report from the Nurses Health Study II, a cohort of 105,716 female registered nurses aged 29 to 46 years old at baseline in 1993. In that cohort the adjusted hazard ratio for breast cancer among women who had one or more induced abortions was 1.01 (95% CI, 0.88–1.17) and the adjusted hazard ratio for breast cancer among women who had one or more miscarriages was 0.89 (95% CI, 0.78–1.01) [10
]. Public concern regarding the relationship between induced abortion and breast cancer risk continues to be voiced despite the mounting evidence that no association exists [11
]. Much of the data prompting this concern came from case-control studies, many of which were affected by bias or design flaws [8
]. More recently published case-control studies have reported no positive association between induced abortion and breast cancer risk [16
]. Prospective cohort studies, which minimize the potential for biased risk estimates, have also found no association between induced abortion and breast cancer risk [5
Our data appear to refute two mechanisms hypothesized to underlie an association of induced abortion to breast cancer risk. The first, that women who undergo abortion do not experience the long-term protection against breast cancer that a full-term pregnancy would provide, is not supported by our observation of similar risk among women whose pregnancies only ended in abortion with women whose pregnancies only ended in full-term live births. The second hypothesis, that the breasts of women undergoing induced abortions are exposed to high hormone levels typical of early normal pregnancy, but then do not experience the terminal cell differentiation that occurs late in a normal pregnancy, leaving breast tissue more vulnerable to carcinogens, is also not supported by our results.
Some women in our cohort may have under-reported induced abortion. The abortion ratio among cohort members was 19.5 per 100 pregnancies ending in induced abortion or live birth for women under age 45 at baseline, as compared with 24.5 per 100 pregnancies in the United States during the same time period [20
]. Socioeconomic status is a strong predictor of all measures of abortion, including the abortion-birth ratio, with figures that are at least 50% lower among women with a college education and those who are not economically disadvantaged [21
]. While CTS participants are highly educated, they represent a range of socioeconomic strata, and thus the lower number of pregnancies ending in induced abortion or live birth for women under age 45 at baseline may not reflect under-reporting. Further, among women in the CTS who were between the ages of 40 and 49 years at baseline, 41.9% reported having had at least one induced abortion, a percentage consistent with Henshaw’s estimate that 43% of women in the United States will have had an induced abortion by the age of 45 years [22
]. The level of under-reporting in our study appears to be low, and in addition, any effect of under-reporting in a prospective cohort study is expected to be non-differential since at the time they reported their reproductive histories, women were not aware of a future breast cancer diagnosis. It is unlikely that breast cancers in the population of women in this study would be undiagnosed, as the rate of screening in our study was very high. Ninety-four percent of women 40–49 years of age at baseline and 97% of women 50+ years of age at baseline reported having had at least one mammogram. The proportions of women in those two age groups who reported having had a mammogram in the two years prior to baseline were 82% and 91%, respectively [12
Another potential limitation pertinent to this analysis, is that data on abortion, miscarriage and tubal pregnancies were measured at baseline. It is possible that women could have experienced another event during follow-up, before breast cancer diagnosis. However when we stratify the analysis by age (>50 and 50–79), thereby comparing groups who would be more or less likely to experience an additional event, respectively, we find no difference in our results.
The current results, may have limited generalizability. In addition to limited racial/ethnic diversity relative to the general female population of the United States, the CTS is characterized by a higher level of education and associated characteristics such as later age at first full-term pregnancy. Nevertheless, our results provide further, strong evidence that neither induced abortion nor miscarriage is associated with breast cancer risk, and may help to resolve any remaining uncertainty as to whether such a relationship exists.