Objectives: To compare different normal reference ranges of fetal blood flow velocity in the middle cerebral artery for predicting fetal anemia. Methods: Eight reference ranges of either middle cerebral artery peak or time-averaged mean velocities were compared using the area under the receiver-operating characteristics (ROC) curve for 113 fetal blood samples from 60 women at risk of fetal red blood cell alloimmunization. Results: The areas under the ROC curves of the different ranges were not significantly different but there were marked differences in sensitivity (range, 7.14-91.78%) and specificity (range, 31.25-96.88%) with the currently used cut-offs. Except for Man's range, the best theoretical cut-offs, defined as those having the best sensitivity with the best specificity, differed from those in current use, especially when using time-averaged mean velocity. Conclusions: Any of the previously reported reference ranges perform well in the non-invasive prediction of fetal anemia. However, with the exception of Man's curve, the currently employed cut-offs for predicting fetal anemia should be changed, some of them markedly, in order to provide reliable support for clinical decisions.