Redundant Foramen Ovale Flap Can Mimic Fetal Coarctation of the Aorta on Prenatal Ultrasound

Fetal coarctation of the aorta, a narrowing of the aorta in the region of the arterial ligament, remains one of the most difficult congenital heart defects to diagnose before birth. A case report published in Cardiovascular Ultrasound now highlights a specific trap: a redundant foramen ovale flap, an overly long flap of the fetal atrial septum, can imitate the hemodynamic signature of true coarctation so closely that experienced centers misdiagnose it. The report follows two fetuses diagnosed with coarctation of the aorta at other hospitals whose diagnoses were later revised, and who were found to have normal hearts after birth. The authors, Qin Lin, Kexuan Liu, Jiangli Dong, Jiangyi Zhao, Zhu Ouyang and Ganqiong Xu, use the two cases to dissect why prenatal ultrasound can mistake this benign anatomical variant for a serious outflow obstruction, and to outline the differential diagnosis between a redundant foramen ovale flap and genuine coarctation.

The stakes of this diagnostic confusion are considerable. According to the report, the prevalence of coarctation of the aorta is 4 per 10,000, and the defect accounts for 8 percent of congenital heart diseases. Accurate prenatal diagnosis is still difficult: less than one-third of cases reported in the literature have been detected by prenatal ultrasound screening, and affected newborns may require immediate surgery after birth. A false positive diagnosis, by contrast, exposes a pregnant woman to anxiety, repeated examinations and, in some settings, over-treatment, while a false negative can leave a neonate undiagnosed until the obstruction becomes clinically apparent. The authors emphasize that the diagnosis of coarctation should be made cautiously precisely because both kinds of error carry real consequences for families and for perinatal care planning.

The two cases at the center of the report were both diagnosed with coarctation of the aorta in other hospitals, and the diagnosis was subsequently revised at the authors’ institution. The report tracks both fetuses through to their postnatal outcomes. Postpartum echocardiography and follow-up showed no abnormalities in either child. The authors attribute the prenatal false positives to the redundant foramen ovale flap, which in these fetuses generated ultrasound findings that overlapped with those used to predict coarctation, including left and right ventricular disproportion and abnormalities of the measured ratios that are normally taken as warning signs of a narrowed aortic isthmus.

The mechanism the authors propose is hemodynamic. In their cases, the foramen ovale flap was so long that it occupied most of the inflow tract of the left ventricle and restricted the bloodstream entering that chamber. When a redundant flap simulates the hemodynamic changes of coarctation and is accompanied by aortic isthmus stenosis, they write, it becomes easy to misdiagnose the fetus as having true coarctation. Using the atrial septal excursion index and the ratio of left to right ventricular size in the classification of Vena and colleagues, both cases fell into type III redundant foramen ovale flap: the excursion index was greater than 0.65 at the end of systole on the apical four-chamber view, with left and right ventricular disproportion and temporary prolapse of the flap to the mitral valve orifice.

The overlap extends to the quantitative parameters that prenatal screening relies on. The sensitivity of a pulmonary artery to aorta ratio greater than 1.6 for predicting coarctation is 86.2 percent, but the specificity is low. Diagnostic accuracy can be improved by combining this with an aortic isthmus to ductal artery ratio below 0.74, an isthmus diameter z-score below minus 2, and evidence of isthmus blood flow disturbance. The problem, the authors note, is that false coarctation caused by a redundant foramen ovale flap can also produce these same ratios. A meta-analysis of prenatal ultrasound diagnostic accuracy cited in the report found that several parameters, including the aortic isthmus diameter z-score, the aortic isthmus to ductal artery ratio, the pulmonary artery to aorta ratio and the presence of a hypoplastic aortic arch, were strongly associated with an increased risk of postnatally confirmed coarctation. Yet when those associations were converted into diagnostic accuracy, ultrasound showed only moderate performance even when all parameters were taken into account.

Several practical factors compound the difficulty. The aortic isthmus lies within the curvature of the aorta, and capturing it is angle-dependent, which can yield artificially small measurements and push the sonographer toward a diagnosis of coarctation. Gestational age, fetal position and maternal condition also influence what the scan shows. A tortuous and rigid aortic arch is the most intuitive sonographic sign of true coarctation; in genuine disease the arch is small and stiff and may even exhibit a so-called stent sign, whereas the arch appearance differs when the apparent narrowing is driven by the flap. In addition, coarctation is a progressive disease in the uterus: abnormalities may be absent in the first trimester and only emerge in the second or third trimester. A redundant foramen ovale flap can exhibit the same temporal evolution, so a scan that appears normal early in pregnancy does not exclude either condition later on.

The postnatal course explains why these prenatal false positives resolve. With the closure of the foramen ovale and the ductus arteriosus after birth, pulmonary blood flow increases, blood flow through the left heart and the aorta rises, and the aorta gradually widens. The isthmus diameter returns to normal, and these neonatal hemodynamic changes overturn the prenatal false positive diagnosis. True coarctation, by contrast, does not return to normal after delivery. In the two fetuses described, the authors suggest that the atrial septal aneurysm observed may itself have formed as a consequence of the redundant flap closing. The practical lesson they draw is that when the fetal aorta and left ventricle are found to be small, coarctation should not be diagnosed blindly if a redundant foramen ovale flap is present, because the findings may represent a temporary abnormality caused by the flap rather than a fixed obstruction.

The authors also set out what a confident prenatal diagnosis of true coarctation should require. Beyond changes in the standard ultrasound ratios, they argue that true coarctation should be accompanied by hypoplasia of the transverse aortic arch, hypoplasia of the left ventricle, and postnatal evidence. They point to emerging imaging techniques, including speckle tracking analysis and fetal cardiac magnetic resonance imaging to assess aortic arch flow, which are now considered potentially useful adjuncts to prenatal diagnosis, and they note that parameters for assessing the aortic arch angle still need further study. Repeated prenatal examination of the fetal heart, combined with early perinatal care, can reduce misdiagnosis by allowing the evolving picture to be followed across gestation rather than judged from a single scan.

The report’s authors are candid about the limits of their evidence. With only two cases, the analysis cannot establish how frequently redundant foramen ovale flap leads to false positive coarctation diagnoses, and they state that attention to this finding and follow-up of affected fetuses should continue in future clinical work. What the cases do demonstrate is a concrete mechanism by which a benign fetal cardiac structure can reproduce the ultrasound profile of a serious defect, and a set of safeguards, careful arch morphology assessment, awareness of angle-dependent measurement error, serial scanning and conservative interpretation of ratio abnormalities when a redundant flap is visible, that may help prenatal teams separate temporary hemodynamic mimicry from genuine aortic narrowing before delivery.

Subject of Research: Prenatal ultrasound differential diagnosis between redundant foramen ovale flap and true coarctation of the aorta

Article Title: Prenatal differential diagnosis of pseudo-COA

Article References: Lin, Q., Liu, K., Dong, J., Zhao, J., Ouyang, Z., & Xu, G. (2026). Prenatal differential diagnosis of pseudo-COA. Cardiovascular Ultrasound, 24(1), Article 13. https://doi.org/10.1186/s12947-025-00352-4

Image Credits: AI Generated

DOI: 10.1186/s12947-025-00352-4

Keywords: coarctation of the aorta, prenatal ultrasound, redundant foramen ovale flap, fetal echocardiography, congenital heart disease, false positive diagnosis, aortic isthmus, atrial septal excursion index, case report, neonatal hemodynamics, fetal cardiac MRI, differential diagnosis