ObjectiveTo determine whether fetal splenic enlargement predicts anemia in Rh-alloimmunized nonhydropic singleton fetuses.MethodsSplenic circumference was measured before funipuncture in 21 singleton pregnancies on 47 occasions. The spleen was imaged in an axial section of the fetal abdomen close to the level used for measurement of the abdominal circumference. The splenic length and width were measured and the circumference calculated by the formula (length and width × 1.57). One measurement per patient was used for each analysis. Splenic circumference was measured and expressed as multiples of the normal median (MoM) for gestational age. One hundred twenty-one cases were used to provide cross-sectional normative data. The expected median splenic circumference values were derived from a normal group. Fetal anemia was defined as hemoglobin deficit, ie, mean hemoglobin concentration for gestation minus the measured value. Anemia was defined as hemoglobin deficit exceeding 2 g/dL, and severe anemia as hemoglobin deficit exceeding 5 g/dL. Receiver operator characteristics curves for the prediction of anemia using different splenic circumference (MoM) values were constructed.ResultsSplenic circumference was an excellent predictor of severe anemia in cases with no prior transfusion: sensitivity 100% and specificity 94.7% (area under the curve = .97,P< .03). The measurement did not correlate significantly with severe anemia in the group with prior transfusion (area under the curve = .73,P= .19).ConclusionSplenomegaly is sensitive for the detection of severe anemia in nonhydropic Rh sensitized cases without prior transfusion. The splenic enlargement could be explained by extramedullary erythropoiesis.