Birth size and breast cancer risk: re-analysis of individual participant data from 32 studies.

A 0.5-kg increment (about one SD) in birth weight was associated with a statistically significant increase in risk in studies based on birth records (pooled RR = 1.06 [95% confidence interval (CI) 1.02–1.09]; p = 0.002) and a borderline significant increase in those based on parental recalls when the women were children, but not in studies based on adult reports, with statistical evidence of heterogeneity between birth weight data sources ( p = 0.003).

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Categorical and continuous analyses of birth weight stratified by study design revealed a positive trend in risk with increasing birth weight categories in data from cohort, nested case-control, and individually matched case-control studies (albeit only statistically significantly for the latter), but not in data from frequency-matched case-control studies ( Figures S1 and S2 ).

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The association with birth weight disappeared after adjustment for birth length and head circumference, while the association with birth length persisted, and remained of borderline significance, after adjustment for birth weight and head circumference.

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