Background: Transposition of the great arteries (TGA) is a conotruncal abnormality. It is associated with ventriculoarterial discordance with the parallel orientation of the great arteries, in which the aorta arises from the right ventricle to supply the systemic circulation, while the main pulmonary artery arises from the left ventricle to supply the pulmonary circulation. Aim: To analyze the prenatal and postnatal outcomes of fetuses with d-TGA and to determine whether prenatal echocardiography may predict postnatal urgent BAS. Methods: A retrospective study of fetuses with d-TGA, for which fetal echocardiography was performed at our tertiary hospital from January 2018 to May 2023. We assessed the appearance of the septum primum and the foramen ovale (FO) flap in the four-chamber view as to whether the FO had a restrictive appearance during measurement of the diameter of the FO at its maximal angle to the attachment point. Color Doppler was used to detect ventricular septal defects (VSD) and measure its diameter both in the four-chamber view and when visualizing the outlets of the great arteries in the sagittal section of the heart. Results: During the study period, 64 fetuses were diagnosed with d-TGA, which was also confirmed postnatally. Of these, 16 fetuses were excluded due to additional cardiac anomalies or the inability to reach the mother. In total, 48 cases were included in this series. In our study, the FO diameter was significantly decreased in the urgent BAS group, compared with the fetuses without urgent BAS (5.1 mm vs 6.3 mm, p0.05). A cut off of 6 mm for the FO diameter (sensitivity, 73.3%; specificity, 72.2%; area under the curve [AUC], 0.764) and 3.2 mm for the VSD diameter (sensitivity, 75%; specificity, 75%; AUC, 0.728) suggested urgent BAS. The FO diameter and the presence of VSD were independent variables associated with urgent BAS in fetuses with d-TGA (p0.05). Conclusion: Prenatal echocardiography in fetuses with d-TGA provides valuable information to estimate the need for postnatal urgent BAS that would prevent immediate life-threatening complications.