Tetralogy of Fallot (TOF) is a set of 4 changes to the heart that include narrowing of the pulmonary artery (pulmonary stenosis), a hole in the ventricular septum (ventricular septal defect or VSD), the aorta moving towards the right side of the heart (“overriding aorta”), and thickening of the right ventricle (right ventricular hypertrophy). The baby’s symptoms will depend on the severity of the pulmonary stenosis. With severe pulmonary stenosis, less blood will be able to reach the lungs and the baby’s blood oxygen levels will be lower. Sometimes “Tet spells” can occur when there is suddenly less blood going to the lungs, and these can be an emergency. Some babies may have less severe pulmonary stenosis and can have blood oxygen levels that are closer to typical for a baby. Model ID: AB