A birth defect characterized by a hole between the upper two chambers (atria) of the heart. When the baby is in the womb, there must be a connection between the two atrial chambers for normal fetal blood flow. The baby receives oxygen from the mother's placenta and that blood normally flows from one upper chamber of the heart to the other and from there is distributed to the rest of the baby's body, largely bypassing the lungs. After delivery, the blood takes a different path through the lungs. Normally, this connection gradually closes after birth. If it does not close because of a true malformation of the atrial wall (septum), it is called an atrial septal defect. This defect rarely causes problems for the newborn infant, even if it is present in prematurely born infants. An atrial septal defect requires either surgery or a heart catheterization to close the hole and correct the abnormal blood flow through the hole, though if small and not causing problems, they are at times left uncorrected.