Smoking and risk of breast cancer in the Generations Study cohort.

For example, in a large pooled analysis of 14 cohort studies, there was a strong trend with smoking interval before first birth after adjustment for potential confounders that included age at first birth and number of live births ( P = 0.0000002), whereas after stratification by age at first birth, the trends in each stratum were weaker ( P = 0.12, 0.02, and 0.28) [ 8 ], which is suggestive of confounding.

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However, although the HR of pre-menopausal ever smokers was somewhat larger than for post-menopausal ever smokers, the former did not reach statistical significance ( P = 0.088), whereas the latter did ( P = 0.040).

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Among parous women, there was a significant trend in breast cancer risk with interval from starting smoking to birth of first child ( P -trend = 0.013); for an interval of 15+ years, the HR was 1.46 (95% CI 1.18–1.81; P = 0.0005).

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