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Smoking before first childbirth increases risk of breast cancer

Smoking cessation

Smoking cessation

Smoking is associated with a wide range of cancers but until recently there has been a lack of clear evidence linking smoking and the development of breast cancer.

A new study suggests that the reason for this may be that the carcinogenic impact of smoking on the breast varies during different periods of a woman's reproductive life. In later life it may be masked by smoking's antioestrogenic effects but at a young age, before first childbirth, smoking significantly increases the risk of breast cancer.

This study, published in the American Journal of Epidemiology, looked at 906 new cases of breast cancer occurring over 15 years in a cohort of 56,042 American women.

It collected data on the number of pack-years of smoking (the number of packs of cigarettes smoked per day multiplied by the number of years smoked).

After adjusting for age and other known and suspected risk factors for breast cancer, it was found that there was a significant trend of rising breast cancer risk associated with increased pack-years of smoking before, but not after, first childbirth.

Ten or more pack-years of smoking before first childbirth was associated with a 78% increased risk of breast cancer compared with non-smokers.

The researchers conclude that the findings are consistent with biological data, which show that female breast tissue is especially sensitive to tobacco carcinogens before first childbirth. During puberty the female breast undergoes rapid proliferation and is mostly composed of undifferentiated tissue, which differentiates during the first full-term pregnancy.

The lack of a relationship between smoking in later life and breast cancer does not necessarily imply a complete lack of sensitivity of breast tissue. It may also be explained by competition between smoking's carcinogenic effects and its ability to lower oestrogen levels.

The high rate of smoking among young women is a major concern. This study provides further evidence of the benefits of helping young women to quit smoking before they start a family.

Ha M, Mabuchi K, Sigurdson AJ et al. Smoking Cigarettes before First Childbirth and Risk of Breast Cancer. Am J Epidemiol 2007;166:51-61

Reviewer

Dr Kevin Lewis
Former GP, Clinical Director of Smoking Cessation, Shropshire County Primary Care Trust

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