This is the most common type of ASD and is characterized by the presence of a large opening between the right and left atria. This condition can be the result of one of the following two developmental defects:
a) Excessive resorption of septum primum leading to the formation of a very large foramen secundum, which is not completely covered by a normally developed septum secundum.
b) Inadequate development of septum secundum, so that this septum is too short to cover the foramen secundum completely.