Purpose: In high-risk pregnancies, oligohydramnios is frequently used to identify fetuses at risk of an adverse outcome. The purpose of this study was to evaluate the effect of oligohydramnios on perinatal outcome in uncomplicated pregnancies between 40.0 and 41.6 weeks. Methods: From January 1997 to December 2000 all uncomplicated pregnancies reaching 40.0 weeks' gestation with a singleton non-malformed fetus and reliable dating underwent monitoring with serial determination of amniotic fluid index (AFI) and biophysical profile. Labor was induced for AFI 5 cm, biophysical profile score of 6 or less, rise in maternal blood pressure >140/90 mm Hg, or gestational age of 42.0 weeks. Perinatal outcome was compared between cases with AFI 5 cm and those with AFI >5 cm using Chi-square or Fisher's exact test for categorical variables, Student's t-test for continuous variables, and logistic regression analysis. A two-tailed p value <0.05 or an odds ratio (OR) with 95% confidence interval (CI) not inclusive of the unity was considered significant. Results: Three thousand and forty-nine women met the inclusion criteria, 341 of which (11%) had an AFI 5 cm. Gestational age at delivery, rates of nulliparity and induction of labor were significantly different between cases with oligohydramnios and those with normal AFI (all p<0.001). Rates of cesarean delivery for nonreassuring fetal testing (8.2% vs. 3.9%, p<0.001) and of neonates with birth weight <10th percentile (13.2% vs. 5.5%, p<0.001) were significantly higher in the AFI 5 cm group compared with the AFI >5 cm. No significant differences were identified between the two groups in rates of meconium-stained amniotic fluid, 5min Apgar score <7, or umbilical artery pH <7. Logistic regression analysis demonstrated that the association between oligohydramnios and rate of cesarean delivery for non-reassuring fetal testing lost significance after controlling for gestational age at delivery, nulliparity and induction of labor, whereas the association between AFI 5 cm and low birth weight centiles remained statistically significant (OR=2.2, 95% CI 1.5, 3.2). Conclusion: In conclusion, in uncomplicated pregnancies at 40.0 to 41.6 weeks, oligohydramnios is independently associated with a higher risk of low birth weight centile.