Interatrial septal development. The primitive atrium is a single cavity (Panel A) subsequently divided by the septum primum which grows down from the roof of the atrium, toward the developing endocardial cushions (Panel B). Thus, small perforations begin to develop superiorly resulting in the ostium secundum (Panel C). The atrial roof grows down along the right side of the septum primum, the septum secundum, which comes to lie over the ostium secundum, however an opening remains between septa, the PFO (Panel D). At birth, lung pressures drop and the blood pressure in the left atrium exceeds that of the right atrium, so that the septum primum is shoved against the septum secundum, obtaining septa fusion (Panel E). If this final step does not occur, PFO results (Panel F).