First- and Second-Trimester Cardiovascular Anomalies in Trisomy 21 Fetuses: Anatomy, Embryology, Genetics and Imaging.

Background: Trisomy 21 (T21) is strongly associated with congenital heart disease, particularly atrioventricular septal defect (AVSD), ventricular septal defect (VSD), atrial septal defect (ASD) and selected conotruncal and arch anomalies. First- and second-trimester ultrasound, Doppler and fetal cardiac MRI enable increasingly early and detailed characterization of these lesions, while advances in molecular cardiogenesis have linked specific phenotypes to dosage-sensitive genes on chromosome 21. Methods: This narrative review synthesizes contemporary evidence on structural and functional cardiovascular anomalies in T21 fetuses in the first and second trimester, integrating fetal echocardiography, Doppler assessment and fetal cardiac MRI with embryologic and molecular insights, and summarizing trimester-specific detectability and pathophysiologic links to candidate genes in the Down syndrome-critical region. Approximately one quarter to one third of T21 fetuses have major congenital heart disease on high-quality prenatal echocardiography, with AVSD representing about half of all lesions and VSD, tetralogy of Fallot (TOF), arch anomalies, venous return abnormalities and functional markers (increased nuchal translucency, tricuspid regurgitation, ductus venosus abnormalities) comprising the remainder. Results: First-trimester detection relies on functional markers and early four-chamber and outflow-tract views, whereas second-trimester studies refine anatomic definition and hemodynamics, with MRI reserved for complex cases. Overexpression of genes such as DSCAM, COL6A1/COL6A2, DYRK1A and RCAN1 perturbs endocardial cushion, conotruncal and vascular development. Conclusions: Early, protocol-driven cardiac imaging in T21 supports timely diagnosis, risk stratification and multidisciplinary counselling, and links fetal imaging phenotypes with chromosome 21 gene dosage to advance personalized management and future genotype-phenotype research.