Objective The purpose of this study was to determine whether fetuses with a congenital heart defect demonstrate changes in cerebrovascular impedance.Study design Fetal echocardiograms from January 2001 to May 2005 were reviewed. Cases had sonographically diagnosed congenital heart defects; control subjects were gestational age-matched fetuses with normal echocardiograms. The pulsatility index in the middle cerebral artery was used to measure impedance to cerebral blood flow. Abnormal middle cerebral artery pulsatility index was defined as less than the 5th percentile. Cases were subgrouped into mixing versus nonmixing lesions.Results Of 142 total fetuses, there were significantly more abnormal middle cerebral artery pulsatility indices in the cases (5/71) than in the control subjects (0/71; P = .023); all abnormal middle cerebral artery pulsatility indices occurred in the fetuses with admixing cardiac lesions.Conclusion Fetuses with congenital heart defect are significantly more likely to have decreased cerebrovascular impedance. This may represent a marker of cerebral hypoxemia that is due to intracardiac mixing of oxygenated and deoxygenated blood. Theoretically, this hypoxemia may contribute to the cause of abnormal neurologic development in these infants.