FIGURE 15-2 Incidence of fetal-to-maternal hemorrhage during pregnancy. The numbers at each data point represent total volume of fetal blood estimated to have been transferred into the maternal circulation. (Data from Choavaratana, 1997.) volume of fetal blood in the maternal circulation. Fortunately, a large blood loss-true fetomaternal hemorrhage-is rare. In one series of more than 30,000 pregnancies, fetomaternal hemorrhage ::150 mL occurred in 1 per 2800 births (de Almeida, 1994). he prevalence of fetomaternal hemorrhage of at least 30 mL-the volume of fetal blood covered by a standard 300 �g dose of anti-D immune globulin-is estimated to be 3 per 1000 pregnancies (Wylie, 2010).
Selected causes of fetomaternal hemorrhage are shown in Table 15-2. It also may occur with placenta previa, placental chorioangioma, or vasa previa (Giacoia 1997; Rubod, 2007). In each of these circumstances, however, fetomaternal hemorrhage is extremely uncommon if not rare. And, in more than 80 percent of cases, no cause is identified. With significant hemorrhage, the most common presenting complaint is decreased fetal movement (Bellussi, 2017; Wylie, 2010). A sinusoidal fetal heart rate pattern is infrequently seen but warrants immediate evaluation (Chap. 24, p. 464). Sonography may demonstrate elevated MCA peak systolic velocity, and indeed this is reported to be the most accurate predictor (Bellusi, 2017; Wylie, 2010). Hydrops is an ominous inding. If fetomaternal hemorrhage is suspected, an elevated MCA peak systolic velocity or sonographic evidence of hydrops prompts consideration of urgent fetal transfusion or delivery.
One limitation of quantitative tests for fetal cells in the maternal circulation is that they do not provide information regarding hemorrhage timing or chronicity (Wylie, 2010). In general, anemia developing gradually or chronically, as in alloimmunization, is better tolerated by the fetus than acute anemia. Chronic anemia may not produce fetal heart rate abnormalities until the fetus is moribund. In contrast, significant acute hemorrhage is poorly tolerated by the fetus and may cause profound fetal neurological impairment from cerebral hypoperfusion, ischemia, and infarction. In some cases, fetomaternal hemorrhage is identified during stillbirth evaluation (Chap. 35, p. 646).
FIGURE 15-3 Kleihauer-Betke test demonstrating massive fetalto-maternal hemorrhage. After acid-elution treatment, fetal red cells rich in hemoglobin F stain darkly, whereas maternal red cells with only very small amounts of hemoglobin F stain lightly.
Once fetomaternal hemorrhage is recognized, the volume of fetal blood loss should be estimated. he volume is essential to calculate the appropriate dose of anti D-immune globulin if the woman is D-negative, and it may inluence obstetrical management.
The most commonly used quantitative test for fetal red cells in the maternal circulation is the acid elution or Kleihauer-Betke (KB) test (Kleihauer, 1957). Fetal erythrocytes contain hemoglo bin F, which is more resistant to acid elution than hemoglobin