Doppler examination of the middle cerebral artery in anemic fetuses.

In 24 red cell isoimmunized pregnancies the fetal middle cerebral artery mean blood velocity increased with anemia. The blood velocity was not related to fetal blood PO2 and the relation of increased velocity to anemia was not affected by the pulsatility index. These findings suggest that the hyperdynamic circulation is a consequence of decreased blood viscosity.