Isolated left bundle branch block mimicking outflow ventricular septal defect in a fetus.

Bundle branch block (BBB) is impossible to diagnose in a fetus with conventional fetal echocardiography. Isolated left BBB is rare in the neonatal period. Asymmetric left ventricular remodeling in isolated left BBB is secondary to chronic dyschronous activation and relaxation, resulting in thinning of the interventricular septum (IVS). This report describes a case of possible ventricular septal defect diagnosed at 34 weeks of gestation due to significant dropout in the outflow portion of the IVS seen in multiple views secondary to undiagnosed isolated left BBB in a fetus, with postnatal follow-up evaluation.