Background: In spite of the overwhelming evidence of the detrimental effects of smoking on health the tobacco use among women is rising globally and the age of initiation of daily smoking among women seems to have become as young as it is in men. Today, the majority of women who initiate smoking do so before their first childbirth.
Aim:To prospectively examine how many breast cancer cases that could have been prevented if women did not initiate smoking before first birth by calculating the corresponding population attributable fraction (PAF) of breast cancer
Methods:We followed 137,412 women, aged 34-70 years, who completed a baseline questionnaire between 1991 and 2007, through linkages to national registries through December, 2010. We used Cox proportional hazards models to estimate hazard ratios (HRs) and 95% confidence intervals (CIs) while adjusting for relevant confounders
Results:During a mean follow-up of 12 years, 3,157 women developed invasive breast cancer confirmed by histology. Compared with never smokers, the multivariate adjusted HR estimate for passive smokers was significantly increased with 18% (HR=1.18, 95% CI 1.03-1.35) for women overall and with 48% (HR=1.48, 95% CI 1.10-2.00) when the analyses was restricted to non-alcohol drinkers. Compared with parous never smokers, those who had smoked >10 years before their first childbirth had a 24% (HR=1.24, 95% CI 1.04-1.48) overall and non-drinkers had a 90% (HR=1.90, 95% CI 1.01-3.58) increased risk of breast cancer. The PAF of breast cancer due to smoking initiation before first birth was 9, 3% (CI 3.2-14.9) overall and 15, 9% (CI 1.7-28.1) for women diagnosed before the age of 50.
Conclusions: One in eleven breast cancer cases could have been prevented if women did not initiate smoking before first birth