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Associations between birth size, perhaps as a marker of the pre-natal environment, and subsequent breast cancer risk have been identified before, but the findings from epidemiological studies have been inconsistent.

The team re-analysed data from published and unpublished studies to obtain more precise estimates of the extent to which birth size affects the risk of breast cancer later in life and to investigate whether they could be explained by associations with other risk factors. Their findings are published in.

They examined 32 studies, comprising 22,058 breast cancer cases among a total of more than 600,000 women, the large majority of whom lived in developed countries. They found that birth weight was positively associated with breast cancer risk in studies where the size at birth information was based on birth records (although not in those based on adult self-reports, which tend to be less accurate). Analyses of women with data from birth records showed that a 0.5 kg increment in birth weight was associated with an estimated 7% increase in the risk of breast cancer.

Birth length and head circumference were also positively associated with breast cancer risk when studies with data from birth records were analysed. Of the three birth size measures examined, birth length appeared to be the strongest independent predictor of risk.

The estimated magnitude of the birth size association with breast cancer risk was not affected when the effects of established breast cancer risk factors were accounted for.

Isabel dos Santos Silva, Professor of Epidemiology at LSHTM and lead author of the study, commented: "Our study indicates that birth size is a marker of susceptibility to breast cancer in adulthood, at least in developed countries. The birth size - breast cancer association appeared to be largely independent of known risk factors. Little is known on how the pre-natal environment may affect breast cancer risk later in life. Further research is needed to unravel the biological mechanisms underlying the birth size - breast cancer association."