Induced abortion and risk for breast cancer: reporting (recall) bias in a Dutch case-control study.

BACKGROUND In general, no association has been found between spontaneous abortion (naturally occurring termination of a pregnancy) and the risk for breast cancer. With respect to induced abortion (termination of a pregnancy by artificial means), the results have been more inconclusive. A positive association was found in five studies, no association was found in six studies, and a negative association was found in the only cohort study. It is thought that part of the inconsistency of the reported results may be attributable to reporting (recall) bias, since all but two studies on induced abortion used the case-control design and were based only on information obtained from study subjects. In comparison with breast cancer case patients, healthy control subjects may be more reluctant to report on a controversial, emotionally charged subject such as induced abortion. Thus, differential underreporting may be a cause of spurious associations in case-control studies. PURPOSE Our goal was threefold: 1) to evaluate the relationship between a history of induced or spontaneous abortion and the risk for breast cancer in a Dutch population-based, case-control study; 2) to examine reporting bias by comparing risks between two geographic areas (i.e., western regions and southeastern regions in The Netherlands that differ in prevalence of and attitudes toward induced abortion); and 3) to compare reporting bias in data on induced abortion with reporting bias in data on oral contraceptive use. METHODS Data analyzed in this study were obtained from 918 women (20-54 years of age at diagnosis) who were diagnosed with invasive breast cancer during the period from 1986 through 1989 and had been initially enrolled in a population-based, case-control study investigating oral contraceptive use and breast cancer risk. The women resided in one of four geographic areas that were covered by Regional Cancer Registries: two western regions (Amsterdam and West) and two southeastern regions (East and Eindhoven). Each case patient was pair-matched, on the basis of age (within 1 year) and region, with a control subject who was randomly selected from municipal registries that fully covered the Dutch population. Both the case patients and the control subjects were interviewed at home by the same trained interviewer, who used a structured questionnaire. Reporting bias was examined indirectly by comparing risks between the western and the southeastern regions of the country, which differ in the prevalence of and attitude toward induced abortion. Multivariate conditional logistic regression methods for individually matched case-control studies were used to estimate relative risks (RRs). Reported P values are two-sided. RESULTS AND CONCLUSION Among parous women, a history of induced abortion was associated with a 90% increased risk for breast cancer (adjusted RR = 1.9; 95% confidence interval [CI] = 1.1-3.2). Among nulliparous women, no association between induced abortion and breast cancer was found. Neither among parous women nor among nulliparous women was a history of spontaneous abortion related to the risk for breast cancer. The association between induced abortion and breast cancer was stronger in the southeastern regions of the country, which have a predominantly Roman Catholic population, than in the western regions (adjusted RR = 14.6 [95% CI = 1.8-120.0] versus adjusted RR = 1.3 [95% CI = 0.7-2.6], respectively; test of difference between regions, P = .017), suggesting reporting bias. Support for reporting bias as an explanation for the regional differences was also found in data supplied by both study subjects and their physicians on the use of oral contraceptives. In comparison with physicians, control subjects in the southeastern regions underreported the duration of their oral contraceptive use by 6.3 months more than control subjects in the western regions (P = .007)...