Which is the most common type of atrial septal defect?

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The key idea is that atrial septal defects are categorized by where the hole sits in the septum, and one location is far more common than the others. An ostium secundum defect sits in the central part of the atrial septum, at the area of the fossa ovalis. It results from excessive resorption of the septum primum or insufficient development of the septum secundum during embryology, leaving a persistent opening. This type accounts for the majority of ASDs—roughly around three-quarters of cases—so it’s the most typical form seen.

Ostium primum defects are lower on the septum and commonly accompany other AV valve or canal defects, and defects at the coronary sinus are rare and refer to an unroofed coronary sinus. A common atrium implies a near-complete absence of the atrial septum and is also uncommon. Therefore, the ostium secundum defect is the most frequent type.