We sought to identify prenatal predictive factors for perinatal outcome and to estimate fetal hemoglobin (Hb) levels in large fetomaternal hemorrhages (transfused blood volume [TBV] > 20 mL) by performing a case-series study ( N = 32). Perinatal outcome was favorable (F group, N = 22) and poor (P group, N = 10: four fetal deaths, three postnatal deaths, three cases of severe anemia). Median TBV was 25 mL for the F group and 325 mL for the P group ( P < 0.0001) and median Hb concentration at birth was 15 g/dL for the F group and 5 g/dL for the P group ( P < 0.0001). Receiver operating characteristic analysis revealed that a Kleihauer-Betke test value above 2.5% was the best threshold for predicting adverse outcome, with sensitivity of 100% (95% confidence interval [CI] 76 to 100) and specificity of 96% (95% CI 77 to 100). In utero estimated Hb concentration best correlated with Hb level at birth when calculated using TBV corrected for fetoplacental weight ( P < 0.0001, R(2) = 0.874). The value obtained in the Kleihauer-Betke test was a prognostic factor, and TBV corrected for fetoplacental weight was the best biological marker for assessing fetal Hb level.