MISINTERPRETATION OF TRISOMY-18 AS A PSEUDOMOSAICISM AT 3RD-TRIMESTERAMNIOCENTESIS OF A CHILD WITH A MOSAIC 46,XY 47,XY,+3/48,XXY,+18 KARYOTYPE/

False-negative trisomy 18 has been reported after chorionic villus sam pling, but not after amniocentesis. We describe a double aneuploidy in cultured amniocytes that was initially misinterpreted as a pseudomosa icism. A patient was referred at 31 weeks of gestation because of feta l anomalies at ultrasound examination. Karyotyping of amniocytes showe d a 47,XY,+3 karyotype in 61 clones and a 48,XXY, +18 karyotype in one clone. The latter was interpreted as a pseudomosaicism, the more sinc e a second amniocentesis revealed only cells with a 47,XY,+3 karyotype . At 36 weeks gestational age, a boy was born with congenital anomalie s suggestive of trisomy 18. A blood culture showed a 48,XXY,+18 karyot ype, while in fibroblasts a 47,XY,+3/48,XXY,+18 mosaicism was found. U mbilical cord and bladder epithelial tissue also revealed normal 46,XY cells, besides the aneuploid cells. Therefore, the child proper had a 46,XY/47,XY,+3/48,XXY,+18 mosaicism with the clinical symptoms of tri somy 18. To the best of our knowledge, this is the first report of a f alse-negative result of trisomy 18 together with three sex chromosomes after amniocentesis. (C) 1997 by John Wiley & Sons, Ltd.