PRENATAL DIAGNOSIS OF TRISOMY 21 MOSAICISM

Aim: To summarize the experience of prenatal diagnosis of trisomy 21 mosaicism. Methods: A retrospective study which includes seven prenatally detected cases of mosaic trisomy 21, routinely diagnosed among 5837 prenatal investigations performed during a 13-year period (2003-2015) in a single tertiary center. Results: Mosaic trisomy 21 was detected in 0.1% of all prenatal karyotyping analyses performed. Six cases were revealed after amniocentesis and one after chorionic villus sampling. The mean maternal age was 33 years. The proportion of trisomy 21 cells ranged from 4% to 42%. Five out of seven cases (71%) had positive confirmatory studies. Abnormal ultrasound findings, bilateral pyelectasia and cystic hygroma were diagnosed in two cases confirmed to be mosaic for trisomy 21. Pregnancy outcome and postnatal follow-up in two cases with normal karyotypes observed after confirmatory studies were uneventful. Conclusion: Confirmatory studies using amniocentesis and/or cordocentesis should be performed when mosaic trisomy 21 is disclosed, while FISH analysis on uncultured amniocytes is the method of choice in resolving low-level or cryptic mosaicisms. Although fetal outcome is not strictly dependent on the level of mosaicism, an increasing proportion of trisomic cells, and especially the presence of ultrasound anomalies, correlates with high risk of fetal abnormality.