Prenatal Diagnosis of Chromosomal Mosaicism in 18,369 Cases of Amniocentesis

Objective The prenatal diagnosis of chromosomal mosaicism is fraught with uncertainty. Karyotyping, chromosomal microarray analysis (CMA), and fluorescence in situ hybridization (FISH) are three commonly used techniques. In this study, we evaluated these techniques for the prenatal diagnosis of chromosomal mosaicism and its clinical outcome. Study Design A retrospective review of mosaicism was conducted in 18,369 pregnant women from January 2016 to November 2021. The subjects underwent amniocentesis to obtain amniotic fluid for G-band karyotyping with or without CMA/FISH. Cases diagnosed with chromosomal mosaicism were selected for further analysis. Results In total, 101 cases of chromosomal mosaicism were detected in 100 pregnant women (0.54%, 100/18,369). Four were lost during follow-up, 61 opted to terminate their pregnancy, and 35 gave birth to a healthy singleton or twins. Among these 35 cases, postnatal cytogenetic testing was performed on eight and two exhibited mosaicism; however, nothing abnormal was observed in the postnatal phenotype follow-up. Karyotyping identified 96 incidents of chromosomal mosaicism including 13 with level II mosaicism and 83 with level III mosaicism, FISH identified 37 cases of mosaicism, and CMA identified 17. The most common form of chromosomal mosaicism involved monosomy X, of which the mosaic fraction in cultured karyotyping appeared higher or comparable to uncultured FISH/CMA (p < 0.05). Discordant mosaic results were observed in 34 of 101 cases (33.7%), most of which resulted from the detection limit of different techniques and/or the dominant growth of a certain cell line. Conclusion Based on the postnatal follow-up results from the babies born, we obtained a more hopeful result for the prognosis of chromosomal mosaicism. Although karyotyping was the most sensitive method for detecting chromosomal mosaicism, artifacts and bias resulting from culture should be considered, particularly for sex chromosomal abnormalities involving X monosomy, in which the combination with uncultured FISH was necessary. Key Points