Bidirectional passage of minute numbers of cells across the placenta is a physiological event, even though the placenta is considered a barrier separating the maternal and fetal circulations [1,2]. Massive fetomaternal hemorrhage (FMH), however, can result in serious sequelae. This topic will review the clinical presentation, diagnosis, management, and prognosis of pregnancies complicated by massive FMH.
No universally accepted threshold defines the volume of fetal erythrocytes in the maternal circulation that constitutes a massive FMH; volumes of 10 to 150 mL have been proposed . To put this in context, fetoplacental blood volume is approximately 100 mL/kg fetal weight , ranging from 117 mL/kg at 18 weeks to 93 mL/kg at 31 weeks gestation .
A better approach for 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 affecting morbidity and mortality [6,7]. Massive FMH is more likely to be fatal if 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 the fetus is hydropic, which suggests chronic anemia.
FMH >20 to 30 mL at delivery is estimated to occur in approximately 1 in 200 to 300 deliveries [3,8,9]. FMH >80 mL and >150 mL is estimated to occur in 1 in 1000 deliveries and 1 in 5000 deliveries, respectively .