Maternal smoking during pregnancy and offspring IQ

Breslau et al. 1 report that a strong association between maternal smoking during pregnancy and the IQ of the offspring is eliminated by adjustment for various maternal characteristics. The final model (model 4 in tables 5 and 6) includes low birthweight as one of the characteristics adjusted for. Since maternal smoking during pregnancy may affect the prevalence of low birthweight, inclusion of low birthweight in the model may be regarded as possible over adjustment. It would be interesting to know whether the main conclusion would still remain if adjustment was made only for maternal IQ, education, and urban/rural residence and not for low birthweight.