BACKGROUND: Reduced production of the vasodilator nitric oxide (NO) in fetal vessels in pregnant smokers may lower the blood flow to the fetus and result in lower birth weight, length, and head circumference. The present study measured endothelial NO synthase (eNOS) activity in fetal umbilical and chorionic vessels from nonsmokers, smokers, and ex-smokers and related the findings to the fetal outcome. METHODS AND RESULTS: Of 266 healthy, singleton pregnancies, 182 women were nonsmokers, 43 were smokers, and 41 stopped smoking early in pregnancy. eNOS activity and concentration were quantified in endothelial cells of the fetal vessels. Cotinine, lipid profiles, estradiol, l-arginine, and dimethylarginines that may affect NO production were determined in maternal and fetal blood. Serum cotinine verified self-reported smoking. Newborns of smokers had a lower weight (P< or =0.001) and a smaller head circumference (P< or =0.041) and were shorter (P< or =0.001) than newborns of nonsmokers and ex-smokers. eNOS activity in umbilical veins of smokers was 36% lower (P<0.001), eNOS concentration was 47% lower (P<0.001), and the fetal plasma level of high-density lipoprotein was 18% lower (P<0.001) than those of nonsmokers, whereas the same levels were found in umbilical veins from ex-smokers and nonsmokers. The same patterns in eNOS activity and concentration were found in umbilical arteries and chorionic vessels. Fetal plasma levels of estradiol, l-arginine, dimethylarginines, total cholesterol, and triglycerides were similar for nonsmokers, smokers, and ex-smokers. CONCLUSIONS: The findings suggest that maternal smoking reduces eNOS activity in the fetal vascular bed, contributing to retarded fetal growth caused by the reduction of vasodilatory capacity, and suggest that smoking cessation early in pregnancy prevents these effects in newborns.