While a baby grows in the womb, there is a normal opening between the left and right atria (upper chambers) of the heart. If this opening fails to close naturally soon after the baby is born, the hole is called patent foramen ovale (PFO). A patent foramen ovale (PFO) allows blood to bypass the lungs. A baby's lungs are not used when it grows in the womb, so the hole does not cause problems in an unborn infant. The opening is supposed to close soon after birth, but sometimes it does not. In about 1 out of 3 or 4 people, the opening never closes.
The cause of a PFO is unknown. There are no known risk factors.