Every human being starts life with an opening between the two upper chambers of the heart, a small passageway called the foramen ovale. Far from being a defect, this opening is essential to survival in the womb, where the lungs aren’t yet doing their job and blood needs an alternate route to deliver oxygen. In most people the opening seals itself shortly after birth, but in a significant minority it never fully closes, which raises a more interesting question than the one in the title.
Why Every Fetus Needs This Opening
Before birth, your lungs are filled with fluid and aren’t exchanging oxygen. The placenta handles that work instead, delivering oxygen-rich blood through the umbilical vein. But that oxygenated blood enters on the right side of the heart, which in an adult would pump it straight to the lungs. A fetus can’t afford that detour. The foramen ovale solves the problem by letting blood pass directly from the right atrium to the left atrium, bypassing the lungs almost entirely and sending oxygen-rich blood out to the brain and body where it’s needed most.
The foramen ovale isn’t alone in this bypass system. It works closely with another fetal shortcut called the ductus venosus, and together they function as a distribution unit, directing the most oxygen-rich blood preferentially toward the fetal brain and heart.1Wiley Online Library. The fetal circulation When things go wrong in the womb, such as reduced oxygen supply, the fetus compensates by increasing the flow through these shunts, prioritizing blood delivery to the brain. It’s an elegant emergency routing system built into every developing heart.
What Happens at Birth
The moment a newborn takes its first breath, the lungs inflate and blood begins flowing through the pulmonary vessels for the first time. This sudden rush of blood into the left side of the heart raises pressure in the left atrium. Meanwhile, the loss of placental blood flow drops pressure on the right side. That pressure shift pushes a flap of tissue against the foramen ovale like a valve swinging shut, functionally closing the opening within minutes of birth.2PubMed Central. Progressive anatomical closure of foramen ovale in normal neonatal mouse hearts Over the following weeks and months, the tissue gradually fuses and seals permanently in most people. The result is a solid wall between the atria, keeping oxygen-depleted venous blood on the right side separate from the freshly oxygenated blood on the left.
This process isn’t instantaneous or guaranteed. The functional closure, where pressure holds the flap shut, happens quickly. The anatomical closure, where tissue actually grows together and fuses, takes longer and doesn’t always complete.
When the Hole Never Fully Closes
In roughly one in four adults, the foramen ovale’s flap never fuses completely with the surrounding tissue. This is called a patent foramen ovale, or PFO. Estimates of how common it is range from about 15% to 35% of the adult population depending on the study and the detection method used.3PubMed Central. Patent Foramen Ovale-A Not So Innocuous Septal Atrial Defect in Adults A widely cited figure puts it at about 25%.4PubMed. Patent Foramen Ovale and Stroke: A Review
Most people with a PFO never know they have one. The flap still sits against the wall most of the time because left-atrial pressure normally exceeds right-atrial pressure, keeping it pressed closed. But under certain conditions, such as straining, coughing, or bearing down, right-sided pressure can briefly exceed left-sided pressure and push the flap open, allowing a small amount of blood to cross over. For the vast majority of people this is harmless and produces no symptoms whatsoever.
PFO Versus a True Heart Defect
The popular idea of being “born with a hole in your heart” usually conjures images of congenital heart disease, but a PFO and a true atrial septal defect are fundamentally different conditions. An atrial septal defect, or ASD, is an actual gap in the wall between the atria where tissue that should have formed simply didn’t. An ASD allows continuous blood flow between the chambers, leading to right heart volume overload and is often associated with other congenital abnormalities.5PubMed. Patent Foramen Ovale and Atrial Septal Defect A PFO, by contrast, is a remnant of normal fetal anatomy. The tissue formed correctly; it just didn’t fuse completely.
The distinction matters clinically. ASDs generally require monitoring and often surgical correction because the chronic volume overload can enlarge the right side of the heart and eventually cause heart failure or abnormal heart rhythms. A PFO rarely causes those kinds of structural problems because the flap usually stays closed under normal pressure conditions. Echocardiography can distinguish between the two, though they are sometimes confused in casual conversation.6International Journal of Cardiology Congenital Heart Disease. ASD or PFO: State-of-the-art echocardiography says it all
PFO and Stroke Risk
The reason PFOs attract medical attention at all, despite being so common, is their link to a specific type of stroke. In a small number of people, a blood clot that forms in a leg vein or elsewhere in the body can travel through a PFO from the right atrium to the left atrium and from there into the brain’s arteries, causing what’s known as a paradoxical embolism.7PubMed Central. Patent Foramen Ovale and Cryptogenic Stroke: Integrated Management This type of stroke is called cryptogenic because standard workups can’t find a more common cause like atrial fibrillation or severe artery disease.
The connection between PFO and cryptogenic stroke has been studied extensively. A clot that would normally be filtered out by the lungs can slip through the PFO’s flap and enter the arterial system, reaching the brain or occasionally other organs.8PubMed. The pathophysiology of patent foramen ovale and its related complications But it’s worth emphasizing proportions here: roughly a quarter of all adults have a PFO, and only a tiny fraction of them ever have a stroke because of it. Finding a PFO on its own doesn’t mean you’re at elevated stroke risk.
PFO Closure Procedures
For the subset of patients who have had a cryptogenic stroke and whose PFO is identified as the likely culprit, closing the opening is now a well-established option. The procedure is done through a catheter threaded up from a vein in the leg to the heart, where a small device is deployed to seal the flap. It doesn’t require open-heart surgery and patients typically go home the same day or the next.
Long-term data on these closure devices is encouraging. In one series, the device was successfully placed in every patient, with a full occlusion rate that climbed from about 44% on the first day to 71% at two years as tissue grew over the device. Over an average follow-up of just over two years, the annual risk of a recurrent neurological event was under 1%.9PubMed. Percutaneous transcatheter closure of patent foramen ovale in patients with paradoxical embolism More recent trials and longer follow-ups have continued to show that closure outperforms blood thinners alone in preventing repeat strokes in carefully chosen patients.10PubMed Central. A Review of Transcatheter Closure of Patent Foramen Ovale
Guidelines from cardiology societies now recommend PFO closure over antiplatelet therapy alone for adults between 18 and 60 who have had a PFO-associated stroke.11Journal of the Society for Cardiovascular Angiography and Interventions. SCAI Guidelines for the Management of Patent Foramen Ovale For people with a PFO who have never had a stroke, though, closure isn’t recommended. Screening the general population for PFOs wouldn’t make sense given how common they are and how rarely they cause trouble.
Why Scuba Divers Should Care
One group for whom a PFO carries a more concrete everyday risk is scuba divers. During ascent, dissolved gases in the blood can form tiny bubbles. Normally these bubbles travel to the lungs, where they’re filtered out harmlessly. But a PFO gives those bubbles a shortcut directly into the arterial circulation, increasing the risk of decompression illness, which can cause neurological symptoms, joint pain, and in severe cases lasting injury.12PubMed Central. The management of patent foramen ovale in divers: where do we stand?
The numbers bear this out. In a study of 230 divers, about 27% had a PFO, consistent with the general population rate. But 29% of divers with a PFO had experienced at least one major decompression illness event, compared with only 6% of those without one. The risk of major decompression illness was roughly five times higher in the PFO group, and that risk increased with the size of the opening.13European Heart Journal. Risk of decompression illness among 230 divers in relation to the presence and size of patent foramen ovale More recent cohort data has confirmed that divers with a large PFO in particular are more susceptible to decompression illness than previously appreciated and should consider conservative dive profiles or potentially refraining from diving altogether.14PubMed. Decompression Illness in Divers With or Without Patent Foramen Ovale : A Cohort Study
Most recreational divers will never be tested for a PFO unless they experience a decompression event. But for professional divers or people who dive frequently and aggressively, screening is increasingly discussed in diving medicine circles.
The Migraine Connection
Researchers have noticed for years that PFOs seem to be more common in people who get migraines with aura, the kind preceded by visual disturbances or other sensory changes. The proposed mechanism involves tiny blood clots or other particles crossing through the PFO and reaching the brain, where they may trigger cortical spreading depression, the wave of electrical disruption thought to underlie the aura phase. In animal experiments, microemboli injected into the arterial circulation triggered this spreading depression, often without causing any actual tissue damage.15PubMed Central. Microemboli may link spreading depression, migraine aura, and patent foramen ovale
Whether closing a PFO reliably reduces migraines in humans remains an open and somewhat frustrating question. Some trials have shown modest reductions in migraine frequency after PFO closure, but the results have been inconsistent enough that PFO closure is not currently recommended as a migraine treatment. The association is real; the causal chain is still being worked out.
Detecting a PFO
The standard way to look for a PFO is a bubble study, technically called saline contrast echocardiography. A small amount of agitated saline, which contains tiny air bubbles, is injected into a vein while an ultrasound monitors the heart. If bubbles appear on the left side of the heart within a few heartbeats, it means they crossed through an opening like a PFO rather than being filtered out by the lungs. The test is usually performed while the patient bears down or coughs to raise right-sided pressure and coax the flap open.
The test is considered the gold standard, but it’s not perfect. Different clinicians use different criteria for what counts as a positive result, and there are mechanisms that can produce both false-positive and false-negative findings.16PubMed. A critical review of patent foramen ovale detection using saline contrast echocardiography: when bubbles lie Transesophageal echocardiography, where the ultrasound probe goes down the throat for a closer look at the heart, provides better anatomical detail and is often used when a closure procedure is being planned.
Does PFO Run in Families?
There’s evidence that whether your foramen ovale closes after birth may be partly inherited. A study using transcranial Doppler to detect PFOs found that siblings of patients with a PFO were significantly more likely to have one themselves compared with siblings of people without a PFO. The overall odds were about 3.6 times higher. Interestingly, the effect was strongly sex-dependent: among female sibling pairs, the odds ratio jumped to nearly 10, while among males there was no significant difference at all.17PubMed. Is patent foramen ovale a family trait? A transcranial Doppler sonographic study
The reasons for this sex difference aren’t well understood, and the study was relatively small, so the specific numbers should be interpreted cautiously. But the broader point, that PFO persistence has a genetic component, fits with what we know about how variable the closure process is from person to person.
The Heart’s Other Fetal Shortcuts
The foramen ovale isn’t the only bypass that operates before birth and closes afterward. The ductus arteriosus is a blood vessel connecting the pulmonary artery directly to the aorta, providing yet another route for blood to skip the lungs. After birth, rising oxygen levels and falling levels of prostaglandin E2 cause this vessel to constrict and eventually seal shut.18PubMed Central. Molecular and Mechanical Mechanisms Regulating Ductus Arteriosus Closure in Preterm Infants Prostaglandin E2 plays a double role here: it keeps the ductus open during fetal life and its withdrawal after birth helps trigger closure.19PubMed Central. Failure of postnatal ductus arteriosus closure in prostaglandin transporter-deficient mice
When the ductus arteriosus fails to close, the result is called a patent ductus arteriosus, or PDA. This is particularly common in premature infants, whose ductal tissue may not be mature enough to respond fully to the oxygen and prostaglandin signals that drive closure. Unlike a PFO, a PDA usually causes a continuous heart murmur that doctors can hear with a stethoscope, and significant PDAs often need treatment, either with medications that suppress prostaglandins or with a catheter-based or surgical closure.
Both of these fetal shunts, along with a third called the ductus venosus that routes blood past the liver, were described as far back as the second century AD by Galen of Pergamon, who recognized that they were unique to the fetus and closed after birth, even if he didn’t fully understand the circulatory system.20PubMed Central. The three fetal shunts: A story of wrong eponyms
PFO and Physical Performance
If a quarter of adults are walking around with a small opening between their atria, does it affect their ability to exercise? The short answer for most people is no. A study measuring gas exchange efficiency in healthy people with and without PFOs found that while a PFO slightly reduced the efficiency of oxygen transfer at rest, the difference vanished during exercise.21PubMed Central. Effect of a patent foramen ovale on pulmonary gas exchange efficiency at rest and during exercise. The increased blood flow and pressure changes during exertion appear to keep the flap pressed shut in most cases.
There is, however, a small subset of people whose oxygen saturation drops significantly during exercise because of blood shunting through a PFO. In one study, patients who experienced these exercise-related desaturations saw substantial improvement after PFO closure, with oxygen saturation drops improving by an average of about 10 percentage points and functional capacity improving meaningfully as well.22JACC: Cardiovascular Interventions. Clinical Research Provoked Exercise Desaturation in Patent Foramen Ovale and Impact of Percutaneous Closure These cases are unusual, but they illustrate that for a small number of people, a PFO can have real functional consequences beyond stroke risk.
How Marine Mammals Handle the Same Anatomy
Humans aren’t the only mammals born with a foramen ovale, as it’s a feature of all mammalian fetal hearts. But in diving marine mammals, the story gets more interesting. Comparative anatomical studies of whale hearts have found that the tissue around the foramen ovale forms a tunnel- or sleeve-like structure, often with thin threads stretching across the opening. In toothed whales, these threads connect the tissue flap to the wall of the septum, while baleen whales have a more complex network covering the end of the sleeve. Similar structures appear in the hearts of newborn hippopotamuses.23PubMed Central. Comparative anatomy of the foramen ovale in the hearts of cetaceans
The significance of these structures is still debated. Some researchers have speculated that a partially patent foramen ovale in marine mammals could serve a functional role during deep dives, potentially allowing pressure equalization or blood shunting under extreme conditions. Whether this represents an adaptation or simply a byproduct of the same incomplete closure seen in humans remains an open question, but it’s a reminder that the foramen ovale’s story extends well beyond human medicine.