Low-level mosaic trisomy 21 due to mosaic unbalanced Robertsonian translocation of 46,XX,+21,der(21;21) (q10;q10)/46,XX at amniocentesis in a pregnancy associated with a favorable fetal outcome, cytogenetic discrepancy between cultured amniocytes and uncultured amniocytes, cytogenetic discrepancy among various tissues and perinatal progressive decrease of the trisomy 21 cell line

We present prenatal diagnosis of mosaic trisomy 21 at amniocentesis associated with unbalanced Robertsonian translocation in the fetus and a favorable fetal outcome.