The presence of two extra chromosomes in the same individual is rare, there is usually one sex chromosome and one autosomal chromosome aneuploidy. Most commonly, Down syndrome (DS) and Klinefelter syndrome occur together. However, double autosomal trisomies in live born infants are rare with only five cases reported having combinations of chromosomes 8 and 14; 8 and 21; 13 and 18; 13 and 21; and 18 and 21. Double aneuploidy mosaicism involving two different aneuploidy cell lines is even rarer in live born infants with only three cases previously reported. Fernandez Vegas et al. describe a double trisomy case with symptoms of threatened miscarriage at 9 weeks gestation. Chromosome microarray analysis revealed a female chromosome complement with double trisomies 10 and 20. On the other hand, Chen et al. provide evidence that low-level mosaic double trisomy involving trisomy 6 and trisomy 20 at amniocentesis can be associated with a favorable fetal outcome. In the literature, trisomy 20 and 21 has not been reported before in a same patient. Patient 13-years-old female born to healthy non-consanguineous parents, with clinical suspicion and diagnostic analysis of karyotype of 50 metaphases of free trisomy 21 with karyotype 47,xx,+21; history of surgically corrected tetralogy of Fallot and cardiovascular malformation. Patient presents some phenotypic and organic characteristics compatible with DS, however, also presents other symptoms such as heart valve stenosis, incomplete right branch block and recurrent bacterial infections. Therefore, another chromosomal alteration that cannot be observed in the conventional karyotype is suspected, and decided to send a more specialized test, so Array aCGH is requested. A pathogenic duplication was found on chromosome 21q11.2q22.3, which can lead to severe clinical phenotypes, including congenital heart defects, hypotonia, developmental delay, and speech delay, among others. Additionally, a duplication of uncertain significance was found at position 20q13.32. Interstitial duplications of the long arm of chromosome 20 are rare. Previously published cases of isolated trisomy 20q have suggested a new duplication syndrome, but only a few cases have been reported so far, involving approximately seven previously published reports with a total of nine patients. Comparisons between case reports suggest common clinical features of developmental delay and dysmorphic facial features. To date, there is no case report or literature information related to the effects of duplication in 20q13.32 specifically, therefore, it is necessary to determine the effect of said duplication on the genes that are included in the region. The phenotypic heterogeneity in these conditions highlights the need to employ more advanced analyzes to understand these chromosomal pathologies. Therefore, the use of specialized tests such as aCGH not only reveals unusual duplications, such as that of chromosome 20q13.32, but also highlights the importance of understanding their effects on gene expression, including improving the accuracy of prenatal diagnosis. A retrospective cohort study analyzed 772 fetuses, using aCGH for prenatal genetic studies. An 8.3% rate of abnormal pathogenic copy number variations was identified; Therefore, the use of aCGH showed a 4.9% increase in diagnostic performance compared to conventional karyotyping, highlighting its potential to improve the accuracy of prenatal diagnosis. This precision approach to medicine is essential to predict and prevent complications associated with these genetic conditions. In addition, it allows close monitoring to adapt prevention and treatment strategies, promoting participatory and population medicine, improving the management of rare diseases within the paradigm of medical personalization.