The septum secundum, semilunar in shape, grows downward from the upper wall of the atrium immediately to the right of the primary septum and ostium secundum.
Shortly after birth it fuses with the septum primum, and consequently the foramen ovale is closed, but sometimes the fusion is incomplete and the upper part of the foramen remains patent.
The limbus fossae ovalis denotes the free margin of the septum secundum.