The atria are the the upper chambers of the heart. An atrial septal defect (ASD) is a hole in the wall between the left and right chambers of the atria. It is present at birth.

Blood passes from the left atrium to the right atrium in babies born with ASD. This eventually can cause problems in the lungs.

Heart Chambers and Valves
heart anatomy
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ASD is occurs during fetal development. It is present at birth. Some cases may be caused by a genetic defect or abnormality inherited from a parent. Others can be caused by illnesses suffered by the mother during pregnancy.

Most of the time, the cause is unknown.