The purpose of this study is to evaluate the role of maternal oxidative stress in lowering neonatal birth weight. Women (N=261) with singleton pregnancy were analyzed for biomarker levels of oxidative stress after recruitment at the time of hospitalization for delivery in Korea between 2000 and 2001. Among the neonates, 247 births were full-term infants and 14 births were pre-term infants. Biomarkers measured for oxidative stress were maternal urinary 8-hydroxydeoxyguanosine (8-OH-dG) and malondialdehyde (MDA). The women with pre-term infants had higher concentrations of urinary 8-OH-dG and MDA than those with full-term babies. The concentrations of maternal urinary 8-OH-dG and MDA were inversely associated with birth weight of full-term deliveries after adjusting for potential confounders including maternal age, body mass index, dietary intake, alcohol consumption, smoking exposure, occupational status, and neonatal sex (P<0.05). This study demonstrates that increase of 8-OH-dG and MDA concentrations in urine of pregnant women were associated with reduced birth weight in full-term deliveries.