Title: Transposition of Great Arteries in a Foetus with Semilobar Holoprosencephaly at 21 Weeks: A Rare Embryological Association Background & Objective: Holoprosencephaly (HPE) is a complex congenital brain malformation resulting from the incomplete cleavage of the prosencephalon. It is frequently associated with midline craniofacial anomalies and congenital heart defects. However, while septal defects are relatively common, the coexistence of Transposition of the Great Arteries (TGA)—a conotruncal anomaly—is exceptionally rare. This case aims to highlight this unique association and underscore the shared embryological pathways underlying both conditions. Case Presentation: A 21-week gestation foetus presented for routine antenatal ultrasonography. Detailed neurosonography revealed features consistent with semilobar holoprosencephaly. Key findings included partial separation of the cerebral hemispheres, a single monoventricular cavity, and fused thalami, alongside characteristic craniofacial abnormalities. Investigations & Findings: Due to the CNS findings, a comprehensive, targeted foetal echocardiogram was performed. The cardiac evaluation demonstrated clear ventriculoarterial discordance. The aorta was visualized arising anteriorly from the morphological right ventricle, while the pulmonary artery emerged from the left ventricle, confirming the diagnosis of Transposition of the Great Arteries (TGA). No other significant extracardiac anomalies were noted. Discussion: The simultaneous occurrence of semilobar HPE and TGA points toward a shared embryological disruption during weeks 3 to 6 of gestation. This dual presentation suggests a systemic defect involving midline patterning, neural crest cell migration, and left-right axis signaling. Disruption in key molecular mechanisms, specifically the Sonic Hedgehog (SHH) and Nodal signaling pathways, likely plays a critical role in both the forebrain malformation and the cardiac outflow tract morphogenesis seen in this foetus. Conclusion: The coexistence of semilobar holoprosencephaly and TGA is a highly rare prenatal finding that emphasizes the absolute necessity of detailed foetal echocardiography whenever a CNS anomaly is detected. Recognizing these rare associations enhances our understanding of developmental biology, improves prenatal diagnostic accuracy, and is crucial for providing comprehensive genetic counselling and prognostic information to expectant parents. Keywords: * Semilobar Holoprosencephaly Transposition of Great Arteries Foetal Echocardiography Sonic Hedgehog (SHH) Pathway Prenatal Diagnosis MAKE PPT TO PRESENT ON THIS RESEARCH ABSTRACT TO PRESENT IN CONFERENCE