The difference between atrial septal defects and secondary atrial septal defects is mainly the size of the diameter of the defect and whether the location of the defect changes. Atrial septal defects and patent foramen ovale are currently considered to be congenital heart defects, but there are differences in the extent and location of the defects. The main pathogenesis of atrial septal defects is congenital hypoplasia of the atrial septum, resulting in abnormal traffic between the left and right atria, and secondary atrial septal defects are located behind and above the coronary sinus. Atrioventricular septal defects are characterized by two septa emanating from the atrial septum at the 6th or 7th week of embryonic development, the first septum being the primary septum and the second septum being the secondary septum. Failure of the primary septum to fuse with the secondary septum at the fossa ovalis leaves a small fissure called patent foramen ovale. The size of the defect is usually small, usually one to two millimeters in diameter, whereas the size of the atrial septal defect is usually larger, up to seven to eight millimeters in diameter. The location of the atrial septal defect is usually variable, whereas the location of the patent foramen ovale is relatively fixed. If a diagnosis of patent foramen ovale or secondary atrial septal defect is made, aggressive treatment is recommended.