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The heart is divided into four chambers. The septum or wall separates the top two chambers, called the atria. In some cases, a portion of this wall fails to form in the fetal heart. The result is an atrial septal defect (ASD).

All infants have a small opening in their atria right after birth, called a foramen ovale. Sometimes, as the fetal heart forms, lower pressure in the right heart causes the left atrium to send more blood through the foramen ovale, creating a larger than usual hole. While the normal foramen ovale closes shortly after the baby is born, this larger hole may not close and is termed an atrial septal defect.
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