Although antibody titers reflect the strength and the amount of the maternal antibody response, their utility in pregnancy management is limited. Titers provide no information aboutfetal status. In an initial sensitized pregnancy, serial antibodytiter values can assist in determining when the maternal anti-body response is strong enough to represent a risk of fetal anemia. A critical titer is that titer associated with a signif-icant risk for severe fetal hemolytic disease and hydrops fetalis. In most centers, this is between 1:8 and 1:32. If the initial antibody titer is 1:8 or less, the Rh D-negative patient can be monitored with titer assessment every 4 weeks. In a first-sensitized pregnancy, titers are generally performed every 4 weeks. With a history of an affected fetus or infant, titers are not helpful in predicting fetal hemolytic disease and further evaluation is warranted.
Evaluation for possible fetal anemia is usually under-taken in the second trimester, although management may be individualized depending on history and available exper-tise. Traditionally, amniotic fluid assessment of the level of bilirubin has been used as a measure of fetal status and an indirect means of estimating the potential for severe fetal anemia. In the second half of a normal pregnancy, the level of bilirubin in the amniotic fluid decreases progressively, whereas in an affected, isoimmunized patient, the amount of bilirubin detected can deviate significantly. The increase in the amniotic fluid bilirubin in affected pregnancies is a result of fetal urinary excretion of the increased amount of circulating bilirubin. Until recently, serial amnio-centeses were performed to determine the level of biliru-bin in the amniotic fluid, which in turn reflected the severity of fetal anemia.
The current trend in management is the measurement of peak velocity of middle cerebral artery (MCA) flow using Doppler ultrasound. The velocity of flow through the MCA is related to the viscosity of the blood. In the setting of fetal anemia, the blood is less viscous due to fewer cells and, therefore, the velocity of flow increases. Gestational age-specific peak velocity normal curves have been derived and correlated with the fetal hematocrit. The degree of peak velocity elevation above the median for that gestational age can be used to estimate the fetal hematocrit and thus the risk of fetal anemia. Using the peak systolic velocity of the MCA, almost all fetuses with moderate to severe anemia can be identified (Figs. 19.1 and 19.2).
Ultrasound assessment of the fetus is also helpful in detecting severe signs of hemolysis that have resulted in profound fetal anemia. Occasionally, the first presenting signs of fetalhemolysis may be hydropic changes in the fetus including subcutaneous edema, pericardial and/or pleural effusions, and ascites. When these findings are identified and hydrops fetalis is diagnosed, the fetal hematocrit is typically <15%.
Regardless of the methods used to monitor pregnan-cies at risk for fetal anemia, all techniques are designed to determine the fetal hematocrit using indirect measures. If the monitoring test indicates a risk for fetal anemia, or if hydrops is diagnosed, cordocentesis or percutaneous umbilicalblood sampling (PUBS) is performed to directly measure thefetal hematocrit. Under ultrasound guidance, a needle isadvanced into the umbilical vein, a sample of fetal blood removed, and the hematocrit is measured. In general, the average fetal hematocrit is 36% to 44% and, with severe anemia, it is less than 30% (Box 19.1).
In addition to proce-dures to monitor the fetus for anemia, general tests for fetal well-being are indicated in all isoimmunized women with titers above the critical threshold, because the ability of an affected fetus, even if only mildly anemic, to withstand the stresses of pregnancy and labor may be compromised.