ASD as a Congenital Heart Defect An atrial septal defect (ASD) is one of the common forms of congenital heart defects—conditions present at birth due to incomplete development of the heart during fetal life. In congenital heart defect , structural abnormalities alter the normal flow of blood through the heart and lungs. In the case of ASD, the problem lies in the atrial septum, the wall that should fully separate the right and left upper chambers of the heart. When this wall