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Objective. To assess fetal middle cerebral artery (MCA) peak systolic velocity (PSV) in cases of rhesus alloimmunization and to establish whether MCA-PSV is valid for the prediction of fetal anemia. Methods. The study population included 157 pregnant women diagnosed with rhesus alloimmunization. MCA-PSV measurements were obtained within 3 days of blood sampling for estimation of hemoglobin concentration either at delivery or cordocentesis by the same operator and by means of the same ultrasound machine using techniques described previously. To evaluate the measurements of the MCA-PSV as the multiples of median (MoM) for gestation we used original nomograms for various gestational ages derived from a group of 273 normal fetuses between 22 and 40 weeks of gestation, not at risk for anemia. Receiver-operator characteristic (ROC) curves were employed to evaluate the relation of the sensitivity (the true positive rate) and the false positive rate (100% specificity) of different threshold values of the MCA-PSV. Results. The sensitivity of the MCA-PSV was 94.4% in the case of the subgroup of fetuses with severe anemia. The sensitivity of the MCA-PSV test decreased in less anemic fetuses and was 77.3% in the subgroup with moderate anemia and 32% in the subgroup with mild anemia. According to ROC curves, we selected the optimal MCA-PSV threshold values of 1.15, 1.44, and 1.53 MoM for the prediction of mild, moderate, and severe anemia, respectively. Conclusions. MCA-PSV is a significant Doppler index valid for the prediction of moderate and severe fetal anemia. [ABSTRACT FROM AUTHOR]