Fetoplacental blood volume estimation in pregnancies with Rh alloimmunization

Direct intravascular fetal transfusion under ultrasound guidance has become an important method of treating fetal anemia secondary to maternal red cell alloimmunization. An estimate of normal circulating volume would be useful in selecting the volume of donor blood to be transfused to achieve a desired final hematocrit. Thirty-five fetuses between 21 and 35 weeks of gestation underwent 67 direct intravascular transfusions. The fetoplacental volume relative to fetal weight estimated by ultrasound was found to be relatively constant throughout gestation at approximately 100 cm3/kg. The severity of fetal hemolytic disease and its treatment with intrauterine transfusions did not appear to alter the fetoplacental volume. A nomogram for fetoplacental blood volume versus gestational age is described.