Verhulst et al. The association between PCBs and BMI in our cohort seemed to begin early, demon- strated by its association with birth weight and length. Furthermore, maternal smoking modi- fied this effect. This finding is in agreement with a recent study conducted by Sagiv et al. (2007), who also found effect modification by smoking on the association between PCBs and birth weight. After 1 year of age, PCBs were associated with smaller heights. Hertz-Picciotto et al. (2005) found that prenatal PCBs were associated with increased growth in 5-year-old girls but not in boys. Lamb et al. (2006) found that the association between PCBs and child- hood growth among boys depended on the degree of ortho substitution. On the other hand, estimated effects of DDE appeared to manifest later in life, because we found no association with birth size. We did find an association with weight that depended on the child’s age and whether the mother smoked. It is impor- tant to emphasize that there was a trend for DDE effect modification by sex (data not shown), but the present study did not have sufficient power to render this effect sig- nificant. Another prospective study found that increased prenatal DDE concentra- tions were associated with decreased height at 1, 4, and 7 years of age (Ribas-Fito et al. 2006). Karmaus et al. (2002) demonstrated that growth during childhood was signifi- cantly reduced in girls with high DDE con- centrations measured at 8 years of age. They observed no growth effect of DDE in boys. Contradictory results may be attributable to different degrees of exposure, the inclusion of premature children, the age of assessment of anthropometric measurements (prepuber- tal or pubertal children), and the number of assessed confounding variables. Finally, familial influences have been con- firmed as risk factors increasing BMI in chil- dren. The finding that increased birth weight is associated with higher BMI in early child- hood has been previously documented (Li et al. 2007; Whitaker and Dietz 1998), as has the finding that exposure to prenatal smok- ing increases incidence of overweight children (Oken et al. 2008). In conclusion, this study documents that intrauterine exposure to DDE and PCBs is associated with increased BMI during early childhood. The association with PCBs starts from birth, with a negative effect on birth weight and birth length, whereas the associa- tion with DDE is measurable between 1 and 3 years of age, with no effect on birth weight or length. Future prospective studies are needed to confirm these findings, including studies assessing possible mechanisms by which these pollutants could alter energy metabolism. Finally, we recommend more studies inves- tigating the mechanisms of effect modifica- tion by smoking, especially on the influence of maternal weight gain during pregnancy. 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