How frequently does a congenital heart defect go undiagnosed?

Hard question: This depends on who is evaluating. It is highly unusual for an important congenital heart defect to be missed by a pediatric cardiologist. A major diagnostic tool for the diagnosis of congenital heart disease is a careful, focused physical examination. Echocardiography under the direction of a pediatric cardiologist is an important, reliable tool to confirm the diagnosis and establish details.