Fig. 12

In normal embryological development, resorption of the septum primum at the ostium primum and formation of the foramen ovale in the septum secundum occur simultaneously, as illustrated in Fig. 2. In some cases, excessive resorption of the septum primum (drawn in red) or inadequate development of the septum secundum (drawn in blue) or a combination of both occurs. This leads to a larger than normal defect centrally in the interatrial septum at the future fossa ovalis, called an ostium secundum defect (asterisk). F frontal view, L lateral view