At the end of the fifth week of development, the septum secundum grows from the upper wall of the primitive atrium. It grows to the right of the septum primum, which has already started growing. It grows down towards the septum intermedium formed from the endocardial cushions. Before birth, it does not fuse with the septum intermedium, leaving a gap to form the foramen ovale. Shortly after birth, it fuses with the septum primum to form the interatrial septum, and the foramen ovale is closed. The fossa ovalis denotes the free margin of the septum secundum after birth.
