Abstract 2158: Smoking before first childbirth may explain some of the increase in breast cancer diagnosed before 50 years of age
Authors: Gram Inger T. , Little Melissa A. , Lund Eiliv , Braaten Tonje 
Published: May 2015

Abstract
Background: Today, the majority of middle-aged women in Norway are ever (i.e. either former or current) smokers, who started to smoke in their teens. In the current study we prospectively examined the effect of smoking initiation before first childbirth on breast cancer risk and calculated the corresponding population attributable fraction (PAF) of breast cancer in the Norwegian Women and Cancer Study, a nationally representative prospective cohort study.
Material and Methods: We followed 137,142 women, aged 31-70 years, who completed a baseline questionnaire between 1991 and 2007, through linkages to national registries through December, 2010. Questionnaire data included information on lifestyle factors, including lifetime history of smoking. 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. At enrolment, the women were on average 49 years old, 38% were postmenopausal, and 91% were parous with a mean age at first childbirth of 24 years. Altogether, 60% of the women reported to be ever smokers and 10% reported to be non-alcohol drinkers. 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. The corresponding figures for ever smokers was a non-significantly increased risk of 11% (HR=1.11, 95% CI 0.99-1.26) for women overall and of 28% (HR=1.28, 95% CI 0.93-1.77) for non-alcohol drinkers.
For parous ever smokers, those who had smoked &gt;10 years before their first childbirth had a 24% (HR=1.24, 95% CI 1.04-1.48) increased risk of breast cancer overall while non-drinkers had a 90% (HR=1.90, 95% CI 1.01-3.58) increased risk of breast cancer, compared with parous never smokers.
The PAF of breast cancer due to smoking initiation before the first childbirth was 9, 3% (CI 3.2-14.9). For women diagnosed with breast cancer before and after the age of 50 years the corresponding figure was 15, 9% (CI 1.7-28.1) and 7.9% (1.2-14.1), respectively.
In summary, almost one in ten breast cancer cases among middle aged women in Norway could have been prevented if women delayed smoking initiation until after their first childbirth.
Impact: Smoking before first childbirth is an important risk factor for breast cancer that has been emerging during the last decade due to changing smoking history among Norwegian women.
Citation Format: Inger T. Gram, Melissa A. Little, Eiliv Lund, Tonje Braaten. Smoking before first childbirth may explain some of the increase in breast cancer diagnosed before 50 years of age. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2158. doi:10.1158/1538-7445.AM2014-2158