The foramen ovale is a small opening between the two upper chambers of the heart that allows oxygen-rich blood to bypass the lungs during fetal development. Every human heart has one before birth. It typically closes on its own within the first year of life, but in about 25% of adults, it never fully seals.
Why the Fetal Heart Needs a Shortcut
A developing baby’s lungs are filled with amniotic fluid and can’t process oxygen. Instead, the placenta does that job, delivering oxygenated blood through the umbilical cord. That blood travels up through the fetal liver and into a large vein (the inferior vena cava) heading toward the heart. The problem: if that blood followed the normal adult route, it would get pumped to the lungs first, which would be pointless since the lungs aren’t functioning yet.
The foramen ovale solves this by creating a direct passage from the right atrium to the left atrium. Oxygen-rich blood arriving from the placenta flows along the inner wall of the vein and streams toward this opening, slipping straight across to the left side of the heart. From there, it moves into the left ventricle and out through the aorta to supply the brain, heart, and the rest of the developing body. The vast majority of blood bypasses the lungs entirely.
How It Works Structurally
The foramen ovale isn’t just a hole punched through a wall. It’s formed by two overlapping layers of tissue in the wall (septum) between the atria. One layer, called the septum secundum, forms a rigid border around the opening. The other, the septum primum, acts as a thin flap on the left atrial side, functioning like a one-way valve. Before birth, higher pressure on the right side of the heart pushes this flap open, letting blood flow from right to left. The flap design prevents blood from flowing backward.
What Happens at the First Breath
The moment a newborn takes its first breath, the entire pressure system in the heart reverses. Air fills the lungs, blood vessels in the lungs relax and open up, and blood rushes into them for the first time. This dramatically drops the pressure in the right atrium. Simultaneously, all that blood returning from the now-active lungs raises pressure in the left atrium.
This pressure flip pushes the flap of the septum primum firmly against the septum secundum, pressing the foramen ovale shut. Functionally, it closes almost immediately. Over the following months, the two tissue layers gradually fuse together permanently. Full anatomical fusion typically happens by around 12 months of age.
When It Doesn’t Close: Patent Foramen Ovale
In roughly one in four adults, the two tissue layers never fully fuse. This is called a patent foramen ovale, or PFO. Most people with a PFO have no idea they have one. The flap stays pressed closed by normal left-sided heart pressure, so under everyday conditions, no blood crosses through. A PFO is not a heart defect in the traditional sense. It’s an anatomical variation that causes no symptoms in the vast majority of people who have it.
Problems can arise, however, during moments when pressure on the right side of the heart temporarily exceeds the left. This happens during coughing, straining, heavy lifting, or bearing down (a Valsalva maneuver). During those brief pressure spikes, the flap can open slightly, allowing a small amount of venous blood to cross into the arterial side.
PFO and Stroke Risk
The most significant concern with a PFO is the risk of what’s called a paradoxical embolism. If a blood clot forms in a vein (often in the legs), it normally travels to the lungs, where it either gets filtered or causes a pulmonary embolism. But if a PFO opens during a pressure spike, that clot can slip through the flap, enter the left side of the heart, and travel directly to the brain, causing a stroke.
This mechanism is a common cause of unexplained strokes in younger and middle-aged adults. Among patients 60 or younger who have a stroke with no identifiable cause (called a cryptogenic stroke), roughly 50% have a PFO, compared to 25% in the general population. The younger and healthier the stroke patient, the more likely a PFO played a role.
PFO and Migraine
Researchers have also found that PFO is more common in people who experience migraines with aura. The proposed explanation: certain chemicals in venous blood, particularly serotonin, are normally filtered or broken down during their pass through the lungs. When blood bypasses the lungs through a PFO, these substances can enter arterial circulation and reach the brain directly. Tiny clots crossing the opening may also trigger brief disruptions in brain blood flow that set off migraine episodes.
PFO and Scuba Diving
Scuba divers with a PFO face a higher risk of decompression sickness. During ascent, dissolved nitrogen forms tiny gas bubbles in venous blood. Normally these bubbles get trapped and filtered in the lungs. With a PFO, bubbles can cross into arterial circulation and reach the brain, spinal cord, or joints. One prospective study found that divers with a right-to-left shunt had roughly three times the incidence of confirmed decompression sickness compared to divers without one.
That said, the absolute risk per dive remains low, and adopting conservative diving practices significantly reduces it. These include diving shallower, for shorter durations, and using oxygen-enriched breathing gas. Routine screening of all recreational divers for PFO is not currently recommended unless there’s a history of decompression illness.
How a PFO Is Detected
The standard test is a bubble study, a type of echocardiogram where agitated saline (which contains tiny air bubbles) is injected into a vein while you bear down. If a PFO is present, the bubbles cross from the right atrium to the left and show up on ultrasound. The size of the shunt is graded by how many bubbles appear on the left side: fewer than 10 is considered small, 10 to 30 is moderate, and more than 30 indicates a large opening.
Closure Procedures
For people who have had a stroke attributed to a PFO, a catheter-based closure procedure is an approved treatment to prevent recurrence. A small device is threaded through a vein in the leg up to the heart, where it’s positioned across the opening. The device acts as a permanent plug, and heart tissue eventually grows over it. About 59% of PFO closures are performed for stroke prevention. One notable side effect is that roughly 12% of patients develop an irregular heart rhythm (atrial fibrillation) after the procedure, though this is often temporary.
PFO closure is sometimes considered for severe migraines that haven’t responded to medication, though it’s not approved specifically for that purpose. Pooled trial data show that closure reduces monthly migraine days by about three days, compared to about two days with medication alone. The benefit is modest, and guidelines position it as a last resort for refractory cases rather than a first-line option.