between maternal genotype and smoking quantity and (B) the effect size of the association between smoking quantity and birth weight (see Supplementary Material, Fig. S1). We estimated (A) and (B) in the 1649 mothers with data available on offspring birth weight and first trimester smoking quantity, using linear regression, with sex, gestational age and study as covariates. The expected effect size was then estimated by multiplying (A) × (B). We previously used the same approach to investigate associations between a common variant in the FTO gene, BMI and related metabolic traits (25).
