False negative findings at third trimester chorionic villus sampling (C.V.S.)

A discrepancy is reported between the karyotypes of chorionic cells (direct method):46,XX and of cultured amniotic fluid cells: 47,XX, + 18 in a pregnancy of 30 weeks. A stillborn girl, with external signs of trisomy 18 syndrome, was subsequently shown to have a mosaic pattern in both the lymphocytes and the placenta.