BACKGROUND: Prenatal ventricular size discrepancy with disproportionately smaller left ventricle than right ventricle (L-R/VD) can be a marker for important left-sided structural heart disease in the newborn. METHODS: We reviewed the echocardiograms of all fetuses evaluated at our center with L-R/VD from July 1, 2004 to January 1, 2008. RESULTS: Of the 35 fetuses, 20 (57%) had critical arch obstruction and underwent neonatal intervention (group 1); 15 (43%) did not require newborn intervention (group 2). Ratios comparing left with right heart structures were significantly lower in group 1 fetuses compared with group 2 fetuses. Aortic arch measurement <or= 3 mm was the most sensitive, and abnormal direction of atrial level shunting was the most specific measure to predict the need for neonatal aortic arch intervention. CONCLUSION: Ratios expressing the magnitude of L-R/VD, direction of flow at the atrial septum, and measurements of the aortic arch help identify fetuses that will require neonatal intervention.