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ObjectivesDocumentation of semilunar valve growth in fetal transposition of the great arteries (TGA) and the relationship between neo-aortic root (NAoR) dilatation, a cause for postoperative reinterventions after the arterial switch operation (ASO), and pulmonary valve (PV) annulus dimensions prenatally.MethodsThis retrospective multicenter observational study included TGA fetuses suitable for ASO. Semilunar valve annuli pre-ASO and NAoR diameters (post-ASO) were measured. Trends in annulus diameters were analyzed using a linear mixed-effects model and compared with normal values. Prenatal semilunar valve Z-scores were correlated with NAoR diameters post-ASO.ResultsWe included 137 TGA fetuses (35.8% with significant ventricular septal defects [VSDs]). One hundred twenty-one underwent ASO. Fetal TGA-PV diameters were significantly larger than control aortic valve (AoV) and PV annuli from 23 and 27 weeks, respectively, especially when a VSD was present. Fetal TGA-AoV annuli were significantly larger than control AoV and PV annuli from 26 and 30 weeks, respectively.Z-scores of fetal TGA-PV and NAoR diameter at last follow-up correlated significantly (P ConclusionFetal TGA semilunar valve annuli are larger than control annuli, especially when there is a significant VSD. Factors besides postoperative hemodynamics, including fetal anatomy, PV Z-score, prenatal flow, connective tissue properties, and genetics, may influence the risk for late reintervention in these fetuses.What's already known about this topic? Transposition of the great arteries (TGA) is treated with the arterial switch operation (ASO). Neo-aortic root (NAoR) dilatation is seen postoperatively and may require late reintervention.What does this study add? The first serial data on semilunar valve size in fetal TGA in a large cohort are provided. TGA semilunar valve diameters are larger than in normal fetal hearts, giving a larger left ventricular outflow tract. The prenatal pulmonary valve size may predispose to NAoR dilatation post-ASO.