Objective: To evaluate the association of smoking during a woman's first pregnancy, a period of pronounced growth and differentiation of mammary tissue, and her subsequent breast cancer risk.
Methods: In this matched case-control study, we used linked birth certificate and tumor registry data from the New York State Health Department. Cases were 319 women aged 26-45 who were diagnosed with breast cancer in New York State between 1989 and 1995 and who completed a first pregnancy in New York State after 1987 at least one year prior to diagnosis of cancer. Controls were 768 primiparous women matched to cases on county of residence and delivery date. Information on prenatal smoking and other factors characterizing the woman's first pregnancy was obtained from the pregnancy record of each subject, and the association of these factors to breast cancer risk was assessed using conditional logistic regression.
Results: Smoking during pregnancy was associated with increased risk for breast cancer (crude OR = 2.7, 95% confidence interval (CI): 1.1-6.3). Adjustment for maternal age, subject age, race, and education strengthened this association (OR = 4.8, CI 1.6-14.6).
Conclusions: These findings suggest that cigarette smoking during a woman's first pregnancy may increase her risk for early-onset breast cancer.