Pre-pregnancy exposure to cigarette smoking and subsequent risk of postmenopausal breast cancer

PURPOSE: Late age at first full-term pregnancy has been consistently associated with risk of breast cancer (BC), suggesting that pre-pregnancy breast tissue may be more vulnerable to carcinogenic agents than post-pregnancy breast tissue. Cigarette smoking is a source of carcinogens and is associated with slightly elevated risk of BC. We examined the association of cigarette smoking before 1st pregnancy with risk of postmenopausal BC. METHODS: The Iowa Womenʹs Health Study is a prospective cohort study of 55-69 year old women at baseline in 1986. Of the 36,519 women in the cohort at risk, 7095 (19%) and 4186 (11%) initiated smoking before and after first pregnancy, respectively. As of 12/31/1999, 1995 breast cancers have been identified within this subject population via linkage to the Iowa SEER cancer registry. RESULTS: Compared to parous women who never smoked, women who began smoking after their first full-term pregnancy were not at increased risk of BC (multivariate-adjusted RR = 1.0, 95% CI 0.9–1.2). However, women who began smoking prior to their first pregnancy were at slightly elevated risk of BC (RR = 1.2, 1.1–1.4). Risk ratios were similar for all strata of pack-years (pkyrs) prior to first pregnancy: RRs = 1.2, 1.3, 1.3 and 1.2 for <3, 3–4.9, 5–8.9 and greater-or-equal, slanted9 pkyrs, but there was no evidence of a dose response. Risk was similar in women who initiated smoking cigarettes prior to first pregnancy and were currently smoking at baseline (RR = 1.3, 1.1–1.6) and women who had quit smoking (RR = 1.2, 1.0–1.4). 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 BC for women who started smoking prior to their first full-term pregnancy. Given the modest increase, we cannot completely rule out uncontrolled confounding.