# Oval Foramen: Anatomy of the Fetal Heart Shunt The oval foramen (foramen ovale) is a one-way opening in the interatrial septum of the fetal heart that allows oxygen-rich blood returning from the placenta to pass directly from the right atrium into the left atrium, bypassing the fluid-filled, non-functional lungs. After birth, rising left atrial pressure presses the valve-like septum primum against the septum secundum, functionally closing the shunt within minutes to hours, with anatomical sealing over days to weeks that leaves the fossa ovalis as its permanent remnant. This single flap of tissue solves one of the most elegant problems in comparative cardiovascular physiology. A fetus cannot use its lungs, yet it must deliver the most oxygenated blood it has to the brain and heart. The oval foramen is the shortcut that makes this possible. When it fails to seal, the result is a patent foramen ovale (PFO), a defect that is common in people and increasingly recognized in dogs, cats, and horses [1]. Understanding the oval foramen is therefore foundational for anyone studying fetal circulation, congenital heart disease, or the transition from fetal to neonatal life. ## What the Oval Foramen Is and Where It Sits The oval foramen is not a simple hole. It is a tunnel formed by two overlapping septal structures that develop sequentially in the embryonic heart. The **septum primum** is a thin, crescent-shaped membrane that grows downward from the roof of the primitive atrium. As it descends, it leaves a gap called the ostium primum, which later closes, and then a second opening, the ostium secundum, appears in its upper portion. The **septum secundum** is a thicker, more muscular fold that develops to the right of the septum primum. It grows downward but leaves an opening of its own, the oval foramen, in its lower margin. The result is a flap-valve arrangement. The remaining upper part of the septum primum forms a flexible flap that lies to the left of the septal opening. In the fetus, right atrial pressure exceeds left atrial pressure, so blood pushes the flap open and flows right to left. The flap cannot swing the other way because the septum secundum acts as a rigid backing. This is why the oval foramen is described as a one-way valve rather than a hole. The opening sits at the site that will become the **fossa ovalis** after birth, a shallow depression in the right side of the interatrial septum. The rim of the fossa is the **limbus fossae ovalis** (also called the annulus ovalis), which is the persistent edge of the septum secundum. The floor of the fossa is the fused remnant of the septum primum. ### Why the Shunt Matters Fetal circulation is built around a simple rule: the lungs do not oxygenate blood, so blood must be routed away from them and toward the tissues that need oxygen most. The oval foramen is one of three shunts that accomplish this. The other two are the ductus venosus, which diverts umbilical venous blood past the liver, and the ductus arteriosus, which diverts blood from the pulmonary artery into the descending aorta. Oxygenated blood from the placenta travels up the umbilical vein, partially bypasses the liver through the ductus venosus, and enters the right atrium via the inferior vena cava. Because of the shape of the inferior vena cava opening and the orientation of the flap, this stream is directed preferentially through the oval foramen into the left atrium [2]. From there it passes to the left ventricle and out the ascending aorta to the coronary arteries and brain. Blood returning from the superior vena cava, which is relatively deoxygenated, is directed toward the tricuspid valve and the right ventricle, then largely into the ductus arteriosus and the descending aorta. This streaming is not a passive process. Four-dimensional flow magnetic resonance imaging in a healthy human fetus at 32 weeks of gestation has shown that caval blood flow through the oval foramen can be directly visualized and quantified without contrast agents or anesthesia [2]. The technique confirms what physiologists long inferred from indirect echocardiographic measurements: the oval foramen carries a large, preferential stream of well-oxygenated blood. ## Fetal Shunts and Their Postnatal Remnants The table below summarizes the three major fetal shunts, what they bypass, and what they leave behind after birth. | Shunt | Fetal function | Postnatal remnant | Typical closure | |--|--|--|--| | Oval foramen (foramen ovale) | Right-to-left interatrial shunt, bypasses lungs | Fossa ovalis | Functional within minutes to hours, anatomical over days to weeks | | Ductus arteriosus | Right-to-left shunt from pulmonary artery to aorta, bypasses lungs | Ligamentum arteriosum | Functional within hours to days, anatomical over 1 to 3 weeks | | Ductus venosus | Bypasses liver, carries umbilical blood to inferior vena cava | Ligamentum venosum | Functional within minutes to hours, anatomical over days to weeks | The oval foramen is unique among the three because it closes by a mechanical flap mechanism rather than by smooth muscle constriction. The ductus arteriosus narrows in response to rising oxygen tension and falling prostaglandins. The oval foramen closes because the pressure gradient across the atrial septum reverses. ## The Closure Timeline: From First Breath to Fossa Ovalis Closure of the oval foramen happens in two stages that are often confused. **Functional closure** is the immediate cessation of right-to-left flow. **Anatomical closure** is the permanent fusion of the septum primum to the septum secundum. ### Functional Closure With the first breaths, pulmonary vascular resistance falls sharply as the lungs expand and oxygen enters the alveoli. Pulmonary blood flow increases, and more blood returns to the left atrium through the pulmonary veins. At the same time, clamping of the umbilical cord removes the placental circulation from the right atrium. Left atrial pressure rises above right atrial pressure, and the septum primum flap is pushed against the septum secundum. Flow through the oval foramen stops. This functional closure can occur within minutes of birth, but it is not always complete or permanent. In many newborns, small right-to-left shunts can still be detected by echocardiography in the first days of life, especially during crying or straining when right atrial pressure temporarily rises. ### Anatomical Closure Anatomical sealing follows over days to weeks. The septum primum and septum secundum fuse, initially at the margins and then across the entire overlapping surface. The fused area becomes the floor of the fossa ovalis. In humans, complete anatomical closure is typically achieved by the end of the first year, though the exact timing varies. The process is not always perfect. A study of over 1,000 children reexamined at a median age of 5.2 years found that spontaneous closure occurred in 89.9% of those with a neonatally diagnosed PFO [3]. This means roughly one in ten children with a small neonatal interatrial communication still had a detectable residual shunt at preschool age. The same study found that 81.7% of children with a neonatal atrial septal defect (ASD) had spontaneous closure, but ASDs were distinguished from PFOs by defect size of 4 mm or greater, abnormal location, or multiple communications [3]. ### What Determines Whether It Closes Several factors influence whether the oval foramen seals completely. A larger initial opening, a shorter septum primum flap, and higher right atrial pressures after birth all predispose to persistence. In newborns, PFO is more common in those with lower birth weight and younger gestational age [4]. A study of 2,523 newborns found that more than 95% of those with PFO or patent ductus arteriosus (PDA) closed spontaneously by 6 months [4]. ## How the Oval Foramen Is Observed and Assessed In clinical practice, the oval foramen is assessed primarily by echocardiography. The standard views include the subcostal and apical four-chamber views, where the interatrial septum is visualized directly. Color Doppler shows the direction and magnitude of any shunt. In a normal fetus, right-to-left flow across the oval foramen is expected. In a normal neonate, no flow or a tiny left-to-right shunt is expected. Fetal echocardiography evaluates the oval foramen as a complex anatomical and functional system rather than a simple orifice. A 2023 study of 600 uncomplicated pregnancies assessed septum primum morphology and mobility, ostium secundum configuration, and interatrial shunt direction and magnitude on color Doppler [5]. Restrictive or premature closure of the oval foramen was identified in 9 fetuses, an incidence of 1 in 66 pregnancies [5]. This finding is clinically relevant because premature restriction or closure can impair left heart filling and lead to pulmonary hypertension after birth. In neonates, contrast-enhanced transthoracic echocardiography with a bubble study can detect right-to-left shunts across a PFO. This is particularly useful when paradoxical embolism is suspected [6]. The technique involves injecting agitated saline into a peripheral vein and observing bubbles in the left atrium within a few cardiac cycles. Four-dimensional flow MRI is an emerging research tool that can quantify flow across the oval foramen directly [2]. It is not yet standard clinical practice, but it provides a reference standard for validating echocardiographic estimates. ## Comparative Species Notes The oval foramen is a conserved feature of mammalian fetal circulation, but the frequency of persistence after birth varies. ### Humans PFO is present in approximately 25% of adults [7][8][9][10]. Some sources report a range of 27% to 35% in healthy people [11]. Most PFOs are small and open only during Valsalva maneuvers or other activities that raise right atrial pressure. Resting right-to-left shunts are less common, occurring in about 5% of individuals [8]. PFO is clinically significant because it can allow a venous thrombus to pass into the arterial circulation, a phenomenon called paradoxical embolism [12]. Approximately 50% of patients 60 years or younger with an embolic stroke of undetermined source have a PFO, compared with 25% of the general population [7]. PFO is also associated with migraine, decompression illness in divers, and platypnea-orthodeoxia syndrome, a rare condition in which dyspnea and hypoxemia worsen when sitting or standing and improve when lying down [6][13]. ### Dogs PFO appears to be less frequently recognized in veterinary patients than in humans, likely because many affected animals remain clinically asymptomatic [1]. PDA is one of the most prevalent congenital cardiac defects in dogs, and increased pulmonary blood flow from a PDA can alter right-sided cardiac pressures and predispose to persistent or reopened PFO [1]. When a PFO does cause clinical signs, it may contribute to cyanosis through interatrial shunting. ### Cats PFO is reported in cats, though the veterinary literature is sparse compared with human medicine. The same hemodynamic principles apply: a PFO can remain silent or can contribute to right-to-left shunting if pulmonary hypertension develops. ### Horses PFO has been reported in horses, again with limited clinical documentation. The large size of the equine heart and the relatively low resting heart rate may influence the hemodynamics of a small interatrial communication, but the fundamental anatomy and closure mechanism are the same. The comparative narrative review by Barati emphasizes that the interaction between PDA and PFO has received little attention in veterinary neonates [1]. This is an area where clinical awareness is growing but evidence remains limited. ## When Closure Fails: Patent Foramen Ovale A PFO is the persistence of the oval foramen after birth. It is not the same as an atrial septal defect, though the two are often confused. ### PFO vs. ASD A PFO is a flap-like opening that allows shunting only when the pressure gradient favors it. An ASD is a true deficiency of the atrial septum that allows continuous shunting. In newborns, distinguishing the two can be challenging. An algorithm developed for the Copenhagen Baby Heart Study classified interatrial communications as ASD if the defect was 4 mm or greater, abnormally located, or multiple [3]. Using this algorithm, 89.9% of neonatal PFOs and 81.7% of neonatal ASDs closed spontaneously by preschool age [3]. ### Clinical Consequences of PFO Most people with a PFO have no symptoms. The condition is considered a variant of normal by some authors because of its high prevalence in healthy individuals [11]. However, PFO is associated with several clinical conditions: - **Cryptogenic stroke**: A PFO is found in about 50% of young patients with embolic stroke of undetermined source [7]. - **Paradoxical embolism**: A venous clot passes through the PFO into the arterial circulation [12]. - **Migraine**: Some studies suggest an association, though the mechanism is debated [9]. - **Decompression illness**: Divers with a PFO may be at higher risk because nitrogen bubbles can bypass the lungs [13]. - **Platypnea-orthodeoxia syndrome**: A rare condition of position-dependent dyspnea and hypoxemia [6]. The Risk of Paradoxical Embolism (RoPE) score incorporates age, history of stroke or transient ischemic attack, diabetes, hypertension, smoking, and cortical infarct on imaging to predict the likelihood that a PFO caused a stroke [7]. The PFO-Associated Stroke Causal Likelihood (PASCAL) classification combines the RoPE score with echocardiographic criteria such as large shunt and atrial septal aneurysm [7]. ### Premature Closure of the Oval Foramen At the opposite end of the spectrum is premature closure or restriction of the oval foramen in utero. This is a rare condition that can impair left heart filling and lead to pulmonary hypertension after birth. A study of 10 neonates with isolated prenatal closure or restriction found that all presented within 9 hours after birth, nine with central or differential cyanosis and seven with respiratory distress [14]. Echocardiography uniformly revealed persistent pulmonary hypertension of the newborn and significantly impaired left ventricular function [14]. A separate retrospective review of 34 fetuses with premature closure of the oval foramen found that 62% required neonatal intensive care unit admission, 29% required continuous positive airway pressure, and 6% required milrinone [15]. No subjects required inhaled nitric oxide or extracorporeal membrane oxygenation [15]. These findings underscore that the oval foramen must remain open until birth. Too much closure too early is as problematic as failure to close after birth. ## How the Shunt Closes: A Step-by-Step Mechanism The following diagram traces the normal sequence from fetal right-to-left shunting to postnatal closure. ```mermaid flowchart TD A[Fetal right atrium] --> B[Oxygen rich blood from placenta] B --> C[Oval foramen open] C --> D[Left atrium] D --> E[Left ventricle to brain and heart] A --> F[Deoxygenated blood from head] F --> G[Right ventricle] G --> H[Ductus arteriosus to descending aorta] I[First breath] --> J[Pulmonary vascular resistance falls] J --> K[Left atrial pressure rises] K --> L[Septum primum pressed against septum secundum] L --> M[Functional closure] M --> N[Anatomical fusion over days to weeks] N --> O[Fossa ovalis] ``` The key transition is from node K to node L. When left atrial pressure exceeds right atrial pressure, the flap closes. Everything that follows is a matter of time and tissue remodeling. ## Clinical Relevance, Limitations and Common Mistakes The oval foramen is clinically relevant in three main contexts: fetal echocardiography, neonatal transition, and adult congenital heart disease. In fetal echocardiography, assessment of the oval foramen is part of every standard examination. Restrictive or premature closure can be a marker of underlying cardiac pathology and can predict postnatal respiratory distress [5][15]. In neonatal medicine, failure of the normal transition can present as cyanosis, respiratory distress, or persistent pulmonary hypertension [14]. In adult medicine, PFO is a common incidental finding that becomes clinically significant when paradoxical embolism is suspected [7][12]. Several misconceptions are common among students. **Misconception 1: The oval foramen is a hole.** It is a tunnel with a valve. The flap nature explains why shunting is unidirectional in the fetus and why PFOs often shunt only under provocation. **Misconception 2: The oval foramen closes at birth.** Functional closure occurs within minutes to hours, but anatomical closure takes days to weeks. Incomplete anatomical closure is common and usually harmless. **Misconception 3: PFO and ASD are the same.** A PFO is a patent flap valve. An ASD is a true septal defect. The distinction matters for prognosis and management. **Misconception 4: PFO always causes problems.** Most PFOs are asymptomatic. The condition is present in about 25% of adults and is considered a variant of normal by many authors [11]. **Misconception 5: The oval foramen is only relevant in humans.** PFO is reported in dogs, cats, and horses, and the interaction between PDA and PFO is an active area of veterinary research [1]. Individual cases require veterinary assessment. The information here is educational and is not a substitute for veterinary diagnosis or treatment. ## Quick Review - The oval foramen is a right-to-left interatrial shunt in the fetus that bypasses the non-functional lungs. - It is formed by the overlapping septum primum and septum secundum, creating a one-way flap valve. - Functional closure occurs within minutes to hours after birth as left atrial pressure rises. - Anatomical closure occurs over days to weeks and leaves the fossa ovalis. - PFO is present in about 25% of human adults and is reported in dogs, cats, and horses. - Premature closure of the oval foramen in utero is a distinct and serious condition. - The oval foramen is one of three major fetal shunts, alongside the ductus arteriosus and ductus venosus. ## Frequently Asked Questions ### What is the oval foramen? The oval foramen is a fetal opening in the interatrial septum that allows blood to flow from the right atrium to the left atrium, bypassing the lungs. ### When does the oval foramen close? Functional closure occurs within minutes to hours after birth. Anatomical sealing takes days to weeks and leaves the fossa ovalis. ### What is a patent foramen ovale? A patent foramen ovale is the persistence of the fetal opening after birth. It is present in about 25% of human adults and is usually asymptomatic. ### Is a patent foramen ovale the same as an atrial septal defect? No. A PFO is a flap-like opening that shunts only when pressure favors it. An ASD is a true deficiency of the atrial septum with continuous shunting. ### Do dogs and cats get patent foramen ovale? Yes. PFO is reported in dogs, cats, and horses, though it is less frequently recognized than in humans because most affected animals have no clinical signs. ### Can a patent foramen ovale cause problems in animals? Most PFOs are asymptomatic. In some cases, a PFO can contribute to cyanosis or paradoxical embolism, especially when another cardiac defect such as PDA is present. ## Related Articles - [Comparative Anatomy of the Mammalian Heart](/knowledge/veterinary-medicine/veterinary-anatomy-physiology/comparative-anatomy-mammalian-heart) - [Feline Cardiovascular Anatomy and Physiology](/knowledge/veterinary-medicine/veterinary-anatomy-physiology/feline-cardiovascular-anatomy-physiology) - [Bovine Heart Anatomy and Auscultation Landmarks](/knowledge/veterinary-medicine/veterinary-anatomy-physiology/bovine-heart-anatomy-auscultation-landmarks) - [Canine Heart Anatomy: Chambers, Valves, and Conduction System](/knowledge/veterinary-medicine/veterinary-anatomy-physiology/canine-heart-anatomy-chambers-valves-conduction-system) - [Feline Portosystemic Shunt: Diagnosis and Management](/knowledge/veterinary-medicine/clinical-methods/feline-portosystemic-shunt-diagnosis-management) - [Radiographic Monitoring of Ventriculoperitoneal Shunt in Dogs](/knowledge/veterinary-medicine/diagnostic-imaging/radiographic-monitoring-ventriculoperitoneal-shunt-dogs) ## Sources 1. 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