Pregnancy termination in relation to risk of breast cancer.

OBJECTIVE

To evaluate the association between pregnancy terminations and risk of breast cancer.

DESIGN AND SETTING

Population-based case-control study in Wisconsin, Massachusetts, Maine, and New Hampshire.

STUDY PARTICIPANTS

Cases were women younger than 75 years with a new diagnosis of breast cancer (n = 6888), identified from statewide tumor registries. Controls younger than 65 years (n = 9529) were randomly selected from lists of licensed drivers, or for older subjects, from lists of Medicare beneficiaries. EXPOSURES AND OUTCOMES: Breast cancer risk in relation to spontaneous or induced abortions.

RESULTS

After adjustment for parity, age at first birth, and other risk factors, pregnancy termination (induced or spontaneous) was associated with a relative risk (RR) of breast cancer of 1.12 (95% confidence interval [CI], 1.04 to 1.21), compared with the risk among women who had never had a termination. Induced terminations were associated with a RR of 1.23 (95% CI, 1.00 to 1.51), which was somewhat greater than the risk associated with spontaneous terminations (RR, 1.11; 95% CI, 1.02 to 1.20). The association with induced abortions was stronger for those performed before legalization of abortion in 1973 (RR, 1.35; 95% CI, 1.01 to 1.80) than after this time (RR, 1.12; 95% CI, 0.84 to 1.49), suggesting a bias in reporting this sensitive procedure.

CONCLUSIONS

A weak positive association was observed between abortion--whether induced or spontaneous--and risk of breast cancer. The increase in risk of breast cancer was somewhat greater among women with a history of induced terminations. However, this association may be due to reporting bias and was not significantly different than the slight risk for spontaneous terminations.