Abortions and breast cancer: record-based case-control study.

It has been suggested that abortions leave the breast epithelium in a proliferative state with an increased susceptibility to carcinogenesis. Results from previous studies of induced or spontaneous abortions and risk of subsequent breast cancer are contradictory, probably due to methodological considerations. We investigated the relationship between abortions and subsequent breast cancer risk in a case-control study using prospectively recorded exposure information. The study population comprised women recorded in the population-based Swedish Medical Birth Register between 1973-91. Cases were defined by linkage of the birth register to the Swedish Cancer Register and controls were randomly selected from the birth register. From the subjects' antenatal care records we abstracted prospectively collected information on induced and spontaneous abortions, as well as a number of potential confounding factors. Relative risk of breast cancer was estimated by odds ratios (OR) with 95% confidence intervals (95% CI). A reduced risk of breast cancer was observed for women with a history of at least 1 compared to no abortions (adjusted OR = 0.84, 95% CI = 0.72-0.99). The adjusted OR decreases step-wise with number of abortions to 0.59 (95% CI = 0.34-1.03) for 3 or more compared to no abortions. The patterns are similar for induced and spontaneous abortions. In conclusion, neither a history of induced nor spontaneous abortions is associated with an increased risk of breast cancer. Our data suggest a protective effect of pregnancies regardless of outcome. Copyright 2002 Wiley-Liss, Inc.