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Mean Birth Weight (g) in Relation to Alcohol and Smoking Status1 

Alcohol Drinks/day    Nonsmokers             Smokers   (Packs Per Day)
                                    (22,485)                      (9,119)

                                                                                     pack                 1 pack           1. 5 pk               
  0                        3,468    534 (S.D.)          3,330 548    3,256 533       3,274 519

           <1                        3,500 527                     3,386 522    3,276 532       3,255 548

          1-2                       3,452 582                     3,253 564    3,115 515       3,143 539

           >3                        3,138 512                     3,085 626    3,407 537       3,099 543


The raw data suggests there is no meaningful difference between the crude mean birth weight infants of nonsmokers who drank zero drinks per day compared to <1 drink per day or 1-2 drinks per day.

For three or more drinks, the birth weight trended down significantly even in the nonsmokers.

The authors contend that their multiple regression analysis removes the effect of smoking and shows there is a significant reduction in birth weight for all levels of alcohol intake, even in nonsmokers. This may represent an overestimation of the reliability of multiple regression analysis. For the large group of nonsmokers in this study (22,485 pregnancies), there was no significant difference in crude mean birth weights at low levels of alcohol intake. 

(The additional factor of the tendency of individuals to underreport alcohol intake is difficult to address and correct for as well.)

1.  Mills JL, Graubard BI, Harley EE, et al.  Maternal alcohol consumption and birth weight:  How much drinking during pregnancy is safe?  JAMA 1984:252:1875-1879.