Table 1

 An association between maternal CYP1A1 genotype and low birth weight is only observed in the subgroup of babies born to mothers who smoke during pregnancy5

Maternal smoking status during pregnancyMaternal CYP1A1 genotypeNumber of babies <2500 gNumber of babies ⩾2500 gOdds ratio (p value)*
*p Values calculated using Woolf’s formula for the standard error of the log odds ratio.22
All mothersAA91334
Aa or aa682481.01 (p = 0.972)
Smoking mothers onlyAA1857
Aa or aa22272.58 (p = 0.016)