The case report concerns a male newborn with abnormal prenatal test results and a phenotype characteristic of chromosomal aberrations – trisomy 12 and 21. Diagnosis included prenatal aCGH analysis and amniocyte karyotype, as well as postnatal evaluation of blood lymphocytes and skin fibroblasts. Prenatal testing demonstrated the presence of trisomy 21. Amniocyte karyotyping revealed a male mosaic karyotype with two distinct cell lines: trisomy 12 and 21. Postnatal testing confirmed the finding. The infant exhibited multiple dysmorphic features. The most significant challenges involved the cardiovascular and respiratory systems, including congenital heart defects and severe pulmonary hypertension. This case highlights the crucial role of comprehensive diagnostics, particularly multi-tissue analysis. Managing complex chromosomal aberrations requires a highly individualized therapeutic approach. Documenting rare cases is essential for a better understanding of the clinical spectrum of such disorders