An atrial septal defect (ASD) is a hole in the wall between the two upper chambers of the heart (atria) the left & right Atrium. A hole can vary in size and may close on its own or may require surgery. An atrial septal defect is a type of congenital heart defect- which means a person is born with it. As a baby develops during pregnancy, there are normally several openings in the wall dividing the upper chambers of the heart (atria). These usually close during pregnancy or shortly after birth.