Although the placenta is considered a barrier separating the maternal and fetal circulations, bidirectional trafficking of cells across the placenta is a physiological event . Most investigations have focused upon fetal nucleated blood cells, which have been identified in minute quantities in the maternal circulation throughout normal pregnancy . Sometimes, however, fetomaternal hemorrhage (FMH) involves a large volume of blood. This topic will review the clinical presentation, diagnosis, and management of pregnancies complicated by massive FMH.
There is no universally accepted definition of the volume of fetal erythrocytes in the maternal circulation that constitutes a massive FMH; volumes of 10 to 150 mL have been proposed . Fetal blood volume is about 100 mL/kg fetal weight , ranging from 117 mL/kg at 18 weeks to 93 mL/kg at 31 weeks gestation .
A better approach to assessing the magnitude of FMH is to estimate the percentage of the fetal blood volume represented by the FMH. Looked at in this way, a FMH of 20 mL/kg, which represents 20 percent of the fetoplacental blood volume, is considered massive because it has been associated with significant fetal/neonatal morbidity or mortality (see 'Prognosis' below).
Experiments in sheep show that the rapidity of fetal blood loss is an additional major factor influencing morbidity and mortality [6,7]. Massive FMH is more likely to be fatal if the blood loss occurs over minutes rather than hours, days, or weeks. Although a critical prognostic factor, the rate of fetomaternal bleeding in human pregnancies is generally impossible to assess clinically, unless there are signs of chronic anemia, such as hydrops.
The incidence of FMH greater than 20 to 30 mL at delivery is estimated to be about 1 in 200 to 300 deliveries [3,8,9]. FMH greater than 80 mL and greater than 150 mL is estimated to occur in 1 in 1000 deliveries and 1 in 5000 deliveries, respectively .