Clinically silent massive fetomaternal hemorrhage: important lessons from an illustrative case.

Massive fetomaternal hemorrhage (FMH) >150 mL is rare and may occur in the absence of high-risk obstetrical events. The significance of FMH in Rh D-negative women is alloimmunization with an increased risk of hemolytic disease of the newborn in subsequent Rh D-positive pregnancies and adverse outcomes for the fetus/neonate. The Kleihauer-Betke (KB) acid elution test is used to quantify fetal erythrocytes in the circulation of Rh D-negative women postpartum and to calculate the dose of Rh immune globulin (RhIG) needed for prophylaxis against alloimmunization. In this case, the KB stain unexpectedly revealed 4.5% fetal cells, a finding consistent with a massive FMH of 225 mL, in the absence of a predisposing cause and clinical signs in the infant. This case underscores the importance of FMH quantification in all Rh D-negative women with Rh D-positive fetuses, uncomplicated pregnancies, and healthy newborns. We discuss factors that can affect KB test performance and caveats in interpretation.