The predictive value of findings of the common aneuploidies, trisomies 13,18 and 21, and numerical sex chromosome abnormalities at CVS: Experience from the ACC UK Collaborative Study

We report 611 non-mosaic and 91 mosaic findings of trisomies 13, 18 and 21, and numerical sex chromosome abnormalities in a series of 20 527 CVS, in t he Association of Clinical Cytogeneticists U.K. Collaborative Study, the ma jority with analysis of both direct preparations and cultured cells. No fal se-positive results were encountered among the 611 non-mosaic cases, making these findings a very reliable indicator of the fetal karyotype. One false -negative case was reported. In contrast, the 91 mosaic abnormalities were unreliable predictors of fetal abnormality. Many were associated with norma l outcomes, but a significant proportion of cases of each individual aneupl oidy proved genuine. Mosaicism for 45,X, and trisomies 13 and 18 was dispro portionately common. 17 of the mosaic cases showed complete discordance bet ween the karyotype from direct preparations and that from cultured cells, A ll would have resulted in either a false-positive or a false-negative findi ng if only one technique had been used. Based on our experience, and that o f others, we believe that the highest level of predictive accuracy using CV S can only be achieved if both direct preparation and cell culture are perf ormed. In addition, we continue to recommend that all pregnancies demonstra ting mosaicism for these aneuploidies at CVS undergo amniocentesis or fetal blood sampling to differentiate between confined placental mosaicism and t rue fetal karyotypic abnormality. Copyright (C) 1999 John Wiley & Sons, Ltd .