The atrial septum is the wall between the two upper chambers of the heart (right and left atria). An atrial septal defect (ASD) is an abnormal hole in this wall. ASD is a heart problem that is present at birth (congenital).
ASDs can happen on their own. Or they can happen in children born with other congenital heart defects. Girls have ADS twice as often as boys. Doctors don't know why this is.