Atrial septal defect (ASD) is a fairly common congenital heart disease in pediatric cardiology. For infants and young children, atrial septal defects can be categorized based on their size into three types: A large atrial septal defect typically refers to a defect larger than 10 millimeters. A medium atrial septal defect generally refers to a defect that ranges from 5 to 10 millimeters. A small atrial septal defect usually refers to a defect that is less than 5 millimeters. Therefore, for an infant or young child, an atrial septal defect measuring 12 millimeters would indeed be considered quite severe.