Red blood cell isoimmunization

In red blood cell isoimmunized pregnancies there is transplacental passage of maternal hemolytic antibodies which cause fetal anemia. The fetus compensates for anemia by hemodynamic adjustments but when the hemoglobin deficit exceeds 6 g/dL hydrops fetalis develops 

Fetal anemia is associated with decreased blood viscosity leading to increased venous return and preload with consequent increase in cardiac output. Fetal anemia is associated with increased arterial and venous blood flow velocities. Measurement of fetal middle cerebral artery peak systolic velocity (MCA-PSV) is a useful method of assessing fetal anemia. A cut-off in MCA-PSV of mean plus 1.5 SD’s can identify nearly all severely anemic fetuses with a low false positive rate (about 15%)

Measurement of MCA-PSV is also useful in the assessment of fetuses with other causes of anemia, such as parvovirus B19 infection, fetomaternal hemorrhage and twin-to-twin transfusion syndrome