A patent foramen ovale (PFO) is a type of heart defect that is present in about 15 to 20 percent of the population. It is a congenital defect that usually causes no symptoms or ill effects.

The foramen ovale is a hole between the two upper chambers of the heart (atria) that forms naturally during gestation. 

Beginning in about the fourth week of pregnancy, tissue flaps begin to form in the upper portion heart, dividing the right and left atrium. The first such tissue barrier to form is known as the septum primum. A small hole develops in this septum primum called the ostium secundum, allowing blood to mix freely between the right and left atria. Soon after, however, a second septum, called the septum secundum, forms on the right atria side of the heart. The septum secundum covers up the original hole in the septum primum, but does not totally divide the developing atria. Instead, this septum secundum has a hole in it. This hole is the foramen ovale.

In the fetal heart, these two septa act as a pressure valve. When the pressure in the right atria exceeds the pressure in the left atria, the septum primum, which is located on the left atria side of the heart, swings open. This action allows blood to flow from the right side of the heart to the left side. At this point in development, the baby's lungs are not functioning. When oxygenated blood is introduced into the right side of the heart, a steady supply of this blood flows from the right side of the heart through the foramen ovale into the left side of the heart. From here, the blood flows out into the general circulation.

After birth, a number of conditions change that alter the pressure gradient. The lungs begin to function and blood can flow into the left atria through the pulmonary veins. At the same time, the pressure in the right side of the heart decreases as blood naturally flows toward the lungs. This change in pressure acts like a hand pressing against a door--it holds the flap of the septum primum closed, thus shutting off the foramen ovale. In response, the flaps begin to naturally fuse, and by age two, the two flaps should be joined, creating the atrial septum as a solid wall.

In about 15 to 25 percent of children, however, the flaps do not properly fuse, resulting in a patent (open) foramen ovale. This condition sometimes occurs in conjunction with other defects of the atrial septum, including a septal aneurysm, an atrial septal defect or a malformation of the tricuspid valve called Ebstein's anomaly.