It's unclear what causes the foramen ovale to stay open in some people, though genetics may play a role.
An overview of normal heart function in a child or adult is helpful in understanding the role of the foramen ovale before birth.
Normal heart function
Your heart has four pumping chambers that circulate your blood:
- The right atrium, the upper right chamber, receives oxygen-poor blood from your body and pumps it into the right ventricle through the tricuspid valve.
- The right ventricle, the lower right chamber, 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 atrium, the upper left chamber, 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 ventricle, the lower left chamber, pumps the oxygen-rich blood through a large vessel called the 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 functioning yet. That means there's no need to pump blood to the lungs. At this stage, it's more efficient for blood to bypass the lungs and use a different route to circulate oxygen-rich blood from the mother to the baby's body.
The umbilical cord delivers oxygen-rich blood to the baby's right atrium. Most of this blood travels through the foramen ovale and into the left atrium. From there the blood goes to the left ventricle, which pumps it throughout the body. Blood also travels from the right atrium to the right ventricle, which also pumps blood to the body via another bypass system.
Newborn baby's heart
When a baby's lungs begin functioning, the circulation through the heart changes. Now the oxygen-rich blood comes from the lungs and enters the left atrium. At this point, blood circulation follows the normal circulatory route.
The pressure of the blood pumping through the heart usually forces the foramen ovale closed. In most people, the opening fuses shut, usually sometime during infancy.