Abstract: Objective To evaluate the value of non-invasive prenatal testing (NIPT) for detecting rare autosomal trisomies in fetuses. Methods We retrospectively analyzed the data of cases with rare autosomal trisomies detected by NIPT in our hospital from January, 2019 to April, 2023. Invasive prenatal diagnostic tests including chromosome karyotype analysis, chromosome microarray analysis, copy number variation sequencing, and fluorescence in situ hybridization were performed in all the cases after clinical counseling, and their test results and pregnancy outcomes were analyzed. Results Among 25 282 women receiving NIPT, 56 (0.22%) were found to have high risks for rare autosomal trisomies in circulating plasma DNA. Trisomy 7 was the most frequently detected trisomy, accounting for 45% of the total cases (25/56), while trisomies 1, 4, 17, and 19 were not detected. Among the 46 cases with genetic results of the fetuses, 10 were identified to have true fetal mosaicism. The overall positive predictive value of NIPT was 22% (10/46) for rare autosomal trisomies, and 10% for trisomy 7 (2/20). Of the 52 cases followed up for pregnancy outcomes, 33 (63% ) cases without fetal mosaicism resulted in normal live births, while 10 had unfavorable outcomes including fetal growth restriction, preterm birth, and maternal complications, and among them fetal growth restriction was the most typical and the earliest condition observed in these cases. Among the 22 followed up cases of non-true mosaicism for trisomy 7, 82% resulted in normal live births. Conclusion NIPT increases the detection rate of true fetal mosaicism but with a low positive predictive value. Most of the cases with non- true mosaicism, particularly trisomy 7, can have favorable outcomes. NIPT can also be useful in identifying causes of fetal growth restriction in the second and third trimesters when invasive prenatal testing does not reveal abnormalities.

Key words: rare autosomal trisomies; noninvasive prenatal testing; true fetal mosaicism; confined placental mosaicism