Ultrasound Characteristics of an Isolated Redundant Foramen Ovale Flap in Fetuses and Its Differential Diagnosis from Aortic Coarctation

Objective: To evaluate the potential utility of the ratio of the maximal diameter of the bulging foramen ovale flap to the left atrial diameter (FOFD/LAD) for distinguishing false-positive prenatal suspicion of fetal coarctation of the aorta (CoA), and to examine its association with fetal cardiac structural parameters. Materials and Methods: This retrospective study included a selected referral cohort of 44 fetuses with prenatal suspicion of CoA, who were classified postnatally into the false-positive prenatal suspicion group (n = 29) or the true CoA group (n = 15), along with 50 gestational age-matched controls. FOFD, LAD, atrial and ventricular diameters, great-vessel diameters, and aortic isthmus Z-scores were measured. Associations were assessed using Spearman rank correlation, and intergroup differences were evaluated using the Kruskal–Wallis test with Bonferroni-adjusted Mann–Whitney U tests. Results: FOFD/LAD was significantly higher in the false-positive prenatal suspicion group than in the true CoA and control groups (all p < 0.001), whereas no difference was observed between the true CoA and control groups (p = 0.059). In the false-positive and control groups, FOFD/LAD was positively associated with RAD/LAD, RVD/LVD, and PAD/AoD (all p < 0.001). Both suspected CoA groups showed higher right-to-left cardiac structural ratios and aortic isthmus Z-scores than controls (all p < 0.001), but these indices did not differ between false-positive prenatal suspicion and true CoA cases. Conclusions: In a selected cohort of fetuses with prenatal suspicion of CoA, an increased FOFD/LAD may reflect the presence of a redundant foramen ovale flap and may serve as a promising adjunctive parameter for distinguishing RFOF-related CoA mimicry from true CoA. However, given the limited sample size and moderate reproducibility, these findings should be considered exploratory and require validation in larger independent cohorts.