A "hole" in the wall that separates the top two chambers of the heart. This defect allows oxygen-rich blood to leak into the oxygen-poor blood chambers in the heart. ASD is a defect in the septum between the heart's two upper chambers (atria). IT IS A CONGENITAL CONDITION. Every child is born with an opening between the upper heart chambers. It's a normal fetal opening that allows blood to detour away from the lungs before birth. After birth, the opening is no longer needed and usually closes or becomes very small within several weeks or months. Sometimes the opening is larger than normal and doesn't close after birth. In most children the cause isn't known. Some children can have other heart defects along with ASD.