The fossa ovalis is the remnant of a thin fibrous sheet that covered the foramen ovale during fetal development.

A flap of tissue called the septum primum acts as a valve over the foramen ovale during that time.

After birth, the introduction of air into the lungs causes the pressure in the pulmonary circulatory system to drop.

A small slit-like valvular opening is occasionally found, at the upper margin of the fossa, leading upward beneath the limbus, into the left atrium; it is the remains of the fetal aperture the foramen ovale between the two atria.

The major changes that are made by the body occur at the first breath (in the case of heart and lung functions) and up to weeks after birth (such as the liver's enzyme synthesis).

[4] When this type of aneurysm occurs in the area of the fossa ovalis, an enlarged pouch is formed.

This type of defect generally works like a flap valve, opening during certain conditions of increased pressure in the chest, such as during strain while having a bowel movement, cough, or sneeze.