Influence of environmental smoke exposure during pregnancy on umbilical blood flow velocity and newborns birthweight was assessed in prospective study among 116 pregnant women between 20 and 24 week of pregnancy. The main aim was to search for a possible correlation between cotinine, an effective marker of smoke exposure, and umbilical blood flow as measured by S/D, RI and PI ratios. This study shows a significant increase of systolic/diastolic velocity ratio of the umbilical artery according with increased cotinine levels, either for active or passive smokers. Increase of S/D ratio > 3.0 in umbilical artery in 20-24 week of pregnancy was negatively correlated with newborns birthweight. The results of this study suggest that active and passive smoking by pregnant women causes a direct increase in the vascular resistance of the placenta and contribute to the decreased of the newborns birthweight associated with smoking.