Partial Trisomy and Partial Monosomy 21 Undetected by NIPT and FISH: A Case Report and Literature Review.

Non-invasive prenatal testing (NIPT) is a screening method that analyzes cell-free DNA (cfDNA) in maternal plasma to detect major aneuploidies. However, NIPT can fail to identify subtle chromosomal aberrations. We report a case in which chromosomal microarray analysis (CMA) revealed partial trisomy and partial monosomy of chromosome 21, despite negative results with both NIPT and fluorescence in situ hybridization (FISH). The neonate presented with characteristic phenotypes consistent with Down syndrome (DS). The same chromosomal imbalance was confirmed by postnatal peripheral blood CMA. This case underscores the limitations of NIPT and FISH in detecting structural chromosomal rearrangements and highlights the importance of incorporating CMA when such abnormalities are suspected. Additionally, it emphasizes the critical role of genetic counseling in supporting informed decision-making during the prenatal diagnostic process.