Pilot Evaluation of ddPCR-Based NIPT for Fetal Trisomy Screening in Advanced-Maternal-Age Pregnancies in Mongolia

Background/Objectives: Advanced maternal age (≥35 years) significantly increases the risk of fetal chromosomal abnormalities, particularly trisomies 21, 18, and 13. Non-invasive prenatal testing (NIPT) based on cell-free fetal DNA (cffDNA) in maternal plasma has substantially improved the accuracy and safety of prenatal screening. Methods: We previously developed and clinically validated a multiplex droplet digital PCR (ddPCR)-based NIPT assay for detecting fetal trisomies 21, 18, and 13 in Mongolia. The assay targeted specific loci on chromosomes 21, 18, and 13, using chromosome 1 as an internal reference. A Z-score threshold > 3 indicated high risk, and all positive results were confirmed by invasive karyotyping. Results: In this study, we collected 74 pregnant women of advanced maternal age and samples were successfully analyzed, with high technical performance (mean > 100,000 accepted droplets per reaction and clear signal separation). Ten high-risk pregnancy cases were identified (eight trisomy 21 and two trisomy 18), all of which showed complete concordance with confirmatory karyotyping. Complete concordance with available reference standard results were observed in this limited cohort. No trisomy 13 cases were detected. Conclusions: This ddPCR-based NIPT assay exhibited excellent diagnostic accuracy and reproducibility in a Mongolian cohort of advanced-maternal-age pregnancies. Its technical simplicity, relatively low cost, and minimal infrastructure requirements make it a promising tool for implementation in resource-limited settings. However, the small sample size limits generalizability, and larger multicenter studies are needed to confirm clinical utility across broader populations, including low-risk pregnancies.