Isoprostanes are markers of free radical-catalyzed lipid peroxidation. Evidence suggests that oxidative stress occurs in pregnancies with fetal growth restriction (FGR). The aim of this study was to analyze F2-isoprostanes in amniotic fluid of FGR pregnancies. We tested the hypothesis that F2-isoprostanes are reliable markers to distinguish FGR pregnancies from normal ones and appropriate-for-gestational-age (AGA) from small-for-gestational-age (SGA) newborns. F2-isoprostanes levels were measured by colorimetric enzyme immunoassay in the amniotic fluid of 77 pregnancies with normal fetal growth (group I) and 37 with FGR (group II). Fetal biometry and Doppler measurements were obtained using an ATL HDI 3000 ultrasound system. Isoprostanes were higher in group II than group I. The ROC curve distinguished group I from group II, showing 100% sensitivity and 88.3% specificity at a cutoff of 94 pg/ml. There were no statistical differences in isoprostanes levels between AGA and SGA newborns in group II. The area under the ROC curve drawn to distinguish AGA and SGA newborns showed a sensitivity of 100% and a specificity of 72.3% at a cutoff of 94 pg/ml. The relative risk index indicated a 8.05 times higher risk of birth weight below the 3rd percentiles in group II than in group I. High isoprostanes concentrations can be detected in the amniotic fluid of FGR pregnancies and the assay of isoprostanes in amniotic fluid is a reliable assessment of fetal oxidative stress. Common use of this predictive marker in obstetrics will improve the ability of clinicians to identify those fetuses who will be born SGA or with a birth weight below the 25th percentile.