It is one of two fetal cardiac shunts, the other being the ductus arteriosus (which allows blood that still escapes to the right ventricle to bypass the pulmonary circulation).

Before it does so, bloodflow from the inferior vena cava wears down a portion of the septum primum, forming the ostium secundum.

In time the septa eventually fuse, leaving a remnant of the foramen ovale, the fossa ovalis.

Oxygenated blood from the placenta travels through the umbilical cord to the right atrium of the fetal heart.

Transesophageal echocardiography is considered the most accurate investigation to demonstrate a patent foramen ovale.