People with genetic defects, such as Down syndrome, often have holes in their hearts or other congenital heart defects, says the National Heart, Lung and Blood Institute. Smoking during pregnancy increases the risk of an infant being born with a hole in the heart, as does having a parent with a congenital heart defect. The cause of a hole in the heart is unknown in most cases.
Holes in the heart occur in the septum, the barrier between the left and right sides of the heart, explains the National Heart, Lung and Blood Institute. These holes can allow oxygenated and deoxygenated blood to mix in the heart, reducing its efficiency. They can occur between either the atria, the chambers that receive blood, or the ventricles, the chambers that pump blood out of the heart.
Holes in the atrial septum can be so small that they don't allow much blood through, and many of these heal as the heart grows, states the National Heart, Lung and Blood Institute. Larger holes or those placed high or low on the atrial septum often cannot heal, however. Children with these defects often show no symptoms initially, but they can cause health problems later in life. Small holes between the ventricles also cause no symptoms and often heal on their own, but larger ones cause symptoms even in children and often require surgery to fix.