Low-level mosaic trisomy 21 at amniocentesis and cordocentesis in a pregnancy associated with a favorable fetal outcome and perinatal progressive decrease of the trisomy 21 cell line

Objective: We present low-level mosaic trisomy 21 at amniocentesis and cordocentesis in a pregnancy associated with a favorable fetal outcome and perinatal progressive decrease of the trisomy 21 cell line. Case Report: A 36-year-old, primigravid woman underwent amniocentesis at 16 weeks of gestation because of advanced maternal age. This pregnancy was conceived by in vitro fertilization and embryo transfer (IVF-ET). Amniocentesis revealed a karyotype of 47,XY,+21 [3]/46,XY [17] (15% mosaicism) and simultaneous array comparative genomic hybridization (aCGH) analysis on the DNA extracted from uncultured amniocytes revealed the result of arr (21) x 2 similar to 3 (X,Y) x 1, consistent with 24.5% mosaicism for trisomy 21. Cordocentesis performed at 21 weeks of gestation revealed a karyotype of 47,XY,+21 [3]/ 46,XY [37] (6% mosaicism). She was referred for genetic counseling at 31 weeks of gestation, and continuing the pregnancy was advised. The parental karyotypes and prenatal ultrasound were normal. At 37 weeks of gestation, a phenotypically normal baby was delivered with a body weight of 2900-g. The karyotypes of cord blood, umbilical cord and placenta were 47,XY,+21 [1]/46,XY [39] (2.5% mosaicism), 47,XY,+21 [10]/46,XY [30] (25% mosaicism) and 47,XY,+21 [22]/46,XY [18] (55% mosaicism), respectively. Quantitative fluorescent polymerase chain reaction (QF-PCR) analysis on the DNA extracted from umbilical cord and parental bloods excluded uniparental disomy (UPD) 21 and revealed a maternal origin of the extra chromosome 21. When follow-up at the age of 2 months, the neonate was normal in phenotype and development. The peripheral blood had a karyotype of 47,XY,+21 [1]/46,XY [39] (2.5% mosaicism), and interphase fluorescence in situ hybridization (FISH) analysis on uncultured buccal mucosal cells revealed 4.7% (5/105 cells) mosaicism for trisomy 21, compared with 0% (5/100 cells) in the normal control. Conclusion: Low-level mosaic trisomy 21 at amniocentesis and cordocentesis can be associated with favorable fetal outcome and perinatal progressive decrease of the trisomy 21 cell line. (c) 2024 Taiwan Association of Obstetrics & Gynecology. Publishing services by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).