At 4 weeks, the septum primum develops from the superior aspect of the common atrium. This grows downwards. The lower edge is concave, thus bordering an orifice called the ostium primum. The ostium primum gradually becomes smaller and smaller and finally is obliterated as the septum primum fuses with the endocardial cushions of the atrioventricular junction. This occurs at the 6th week.

Before obliteration of the ostium primum, an opening develops at the superior aspect of the septum primum- called the ostium secundum.

At this time, another septum grows downwards to the right of the septum primum, again from the superior aspect of the atrium, this time the right atrium. This is the septum secundum. It also has a concave lower border, but this border is directed inferiorly and posteriorly. The concave border borders a foramen called the foramen ovale. The foramen ovale thus has a superior limbus and an inferior limbus.

Thus now, blood flows from the right atrium to the left via the foramen ovale and then via the ostium secundum.

The septum primum acts as a flap valve attached to the atrioventricular junction inferiorly and with a free superior border. The opening above the ostium primum is the ostium secundum. To the right of the septum primum is the septum secundum which has an opening inferiorly called the foramen ovale.

After birth, when lung flow is established, left atrial pressure rises and closes the septum primum against the superior limbus of the foramen ovale to form an intact interatrial septum. Fibrosis occurs to fuse these two structures in 75% cases. In 25%, there is no fibrosis and hence whenever the right atrial pressure rises above the left atrial pressure, the flap valve (septum primum) can be pushed to the left to create an opening- these patients are said to have a valve competent patent foramen ovale.