Causes

It's unclear what causes the foramen ovale to stay open in some people, though genetics may play a role.

Normal heart function after birth

Understanding how the heart works may help you understand the role of the foramen ovale before birth.

Your heart has four chambers that pump blood:

• The right upper chamber (right atrium). The right atrium receives oxygen-poor blood from your body and pumps it into the right ventricle through the tricuspid valve.
• The right lower chamber (right ventricle). The right ventricle pumps the blood through a large vessel called the pulmonary artery and into the lungs, where the blood is resupplied with oxygen and carbon dioxide is removed from the blood. The blood is pumped through the pulmonary valve, which closes when the right ventricle relaxes between beats.
• The left upper chamber (left atrium). The left atrium receives the oxygen-rich blood from the lungs through the pulmonary veins and pumps it into the left ventricle through the mitral valve.
• The left lower chamber (left ventricle). The left ventricle pumps the oxygen-rich blood through the body's largest blood vessel (aorta) and on to the rest of the body. The blood passes through the aortic valve, which also closes when the left ventricle relaxes.

Baby's heart in the womb

Because a baby in the womb isn't breathing, the lungs aren't working yet. That means there's no need to pump blood to the lungs. At this stage, blood bypasses the baby's lungs and uses a different path to move oxygen-rich blood from the mother to the baby's body.

The oxygen-rich blood travels from the placenta to the baby's body through the umbilical cord. In the baby's body, blood vessels connect to the umbilical cord. Oxygen-rich blood travels to the heart through the vein that drains blood from the body to the right atrium (inferior vena cava). That blood is directed across the foramen ovale and into the left atrium. From there the blood enters the left ventricle, which pumps it throughout the body.

Newborn baby's heart

When a baby's lungs begin working, blood flow through the heart changes. Now the oxygen-rich blood comes from the lungs and enters the left atrium. At this point, blood flow follows the normal route.

The pressure of the blood pumping through the heart usually forces the flap opening of the foramen ovale to close. In most people, the opening eventually closes, usually sometime during infancy.