Mosaic triple trisomy in amniocytes from a phenotypically and karyotypically normal fetus.

We report the detection of a mosaic triple trisomy, 46,XY/49,XY,+13,+20,+21, in two amniotic fluid specimens obtained from a pregnancy that yielded a normal infant. Traditional cytogenetic methods failed to detect the abnormal cell lineage in fetal blood, foreskin, amnion, umbilical cord, and three different biopsies of the chorion. In addition, fluorescence in situ hybridization study of cells from a buccal smear showed no evidence of cells with three copies of chromosome 20.