OBJECTIVE We present mosaic double trisomy involving trisomy 7 and trisomy 20 at amniocentesis in a pregnancy with a favorable outcome. METHODS A 41-year-old woman underwent amniocentesis at 16 weeks of gestation because of advanced maternal age. Amniocentesis revealed a result of 48,XY,+7,+20[6]/46,XY[26] in cultured amniocytes. At 19 weeks of gestation, repeat amniocentesis was performed, which revealed a result of 48,XY,+7,+20[4]/46,XY[21] in cultured amniocytes. Simultaneous molecular cytogenetic analyses on uncultured amniocytes at repeat amniocentesis revealed no genomic imbalance in array comparative genomic hybridization (aCGH) analysis, no trisomy 7 and no trisomy 20 signals in 114/114 cells in interphase fluorescence in situ hybridization (FISH) analysis, and no uniparental disomy (UPD) 7 and no UPD 20 in quantitative fluorescent polymerase chain reaction (QF-PCR) analysis. Interphase FISH analysis on cultured amniocytes revealed double trisomy of trisomy 7 and trisomy 20 in 5/105 cells (4.7%) compared with 0/100 cells (0%) in the normal control. Prenatal ultrasound findings were unremarkable. The parental karyotypes were normal. The woman decided to continue the pregnancy, and a healthy 2880-g phenotypically normal male baby was delivered at 34 weeks of gestation without any structural abnormality. The cord blood had a normal karyotype. Interphase FISH analysis of the urinary cells revealed no trisomy 7 and no trisomy 20 signals in 51/51 urinary cells. CONCLUSIONS Cytogenetic discrepancy between cultured amniocytes and uncultured amniocytes can occur in mosaicism for double trisomy involving trisomy 7 and trisomy 20 at amniocentesis. Molecular cytogenetic analyses such as aCGH, FISH and QF-PCR on uncultured amniocytes are useful for rapid distinguishing true mosaicism from pseudomosaicism under such a circumstance.