OBJECTIVE: To examine the association of cigarette smoking before first pregnancy with risk of postmenopausal breast cancer in a large population-based cohort. PATIENTS AND METHODS: The Iowa Women's Health Study is a prospective cohort study of 55- to 69-year-old women at baseline in 1986. In January 1986, a questionnaire was mailed to 99,826 postmenopausal women to identify risk factors for cancer and other chronic diseases; 41,836 women responded (42.7% response rate). The primary analyses examined the associations among smoking, parity, age at first birth, and postmenopausal breast cancer. RESULTS: Of the 37,105 women in the cohort at risk, 7095 (19%) and 4186 (11%) initialed smoking before and after first pregnancy, respectively, and 2017 breast cancers were identified before December 31, 1999. Compared with parous women who never smoked, women who began smoking after their first full-term pregnancy were not at increased risk of postmenopausal breast cancer (multivariate-adjusted risk ratio, 1.03; 95% confidence interval, 0.83-1.21). However, women who began smoking before their first pregnancy had a slightly elevated risk of breast cancer (risk ratio, 1.21; 95% confidence interval, 1.07-1.37). Results were not attenuated by adjustment for age at first pregnancy or number of live births. CONCLUSION: These data suggest that cigarette smoking is associated with a slightly greater risk of postmenopausal breast cancer for women who started smoking before their first full-term pregnancy.
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