IGF1 and leukemia: For instance, variation by ethnicity may occur in the association between IGF haplotypes, leukaemia, and birthweight.41 Recent pooled analyses have shown substantial heterogeneity in the association between birthweight and cancer across countries.11,18 In an attempt to diminish the effects of differential birthweight distributions across cohorts, we stratified all models by cohort and performed an analysis taking into account the highest birthweight decile within each cohort.