What is the most common type of atrial septal defect?

The most common type of atrial septal defect (ASD) is the ostium secundum defect. This type of ASD occurs when there is a hole in the atrial septum, located in the centre of the atrial septum, just below the fossa ovalis. Ostium secundum defects make up approximately 70% of all ASDs and are more common in females than in males.