Cytogenetic information on cells from cytotrophoblast, villus mesenchyme, and one or more fetal tissues was available for 192 gestations with mosaicism or non-mosaic fetoplacental discrepancy involving a single autosomal trisomy in the chorionic villus sample (CVS), registered in a collaborative study (EUCROMIC) during the period 1986-1994. In order to identify predictors of confined placental mosaicism (CPM), generalized mosaicism and/or uniparental disomy (UPD), distribution of the mosaic and nonmosaic aneuploid cell lines in the different fetal and extrafetal cell lineages were analyzed. Data were related to existing hypotheses on mechanisms leading to fetoplacental discrepancies and early extraembryonic cell differentiation. Trisomy 21 mosaicism was the one most frequently confirmed in the fetus. Non-mosaic trisomy 13, 18, and 21 in the villus mesenchyme indicated the presence of a trisomic cell line in the fetus proper. Non-mosaic trisomy 2, 7, and 16 in villus mesenchyme was always found with concomitant mosaic or non-mosaic trisomy in the cytotrophoblast, but was never recovered in the fetus. Mosaic trisomy 3, 7, and 20 was predominantly restricted to the cytotrophoblast, mosaic trisomy 2 to the villus mesenchyme. Trisomies 15 and 16 were most often found in both cytotrophoblast and villus mesenchyme and not in fetal cells. This supports the hypothesis that mosaicism/discrepancy for trisomies 15 and 16 results more often than for the other trisomies from trisomic zygote rescue, enhancing their risk for UPD. We recommend, due to the risk of fetal trisomy, amniocentesis in all gestations involving mosaic autosomal trisomy in villus mesenchyme. In gestations with mosaic or non-mosaic autosomal trisomy in both cytotrophoblast and villus mesenchyme we recommend, in order to exclude fetal trisomy and/or UPD, depending on the chromosome involved, further examination by amniocentesis, ultrasound and/or test for UPD. We also recommend, due to a small but not negligible risk of false negative and false positive diagnoses, not to solely use direct preparation.

Of 62 865 karyotyped chorionic villus (CV) samples that were reported to EUCROMIC 1986–1992, 98.5 per cent showed either a normal karyotype (true negative result; 94.8 per cent of the total) or a non‐mosaic chromosomal aberration (true positive non‐mosaic result; 3.7 per cent). True fetal mosaicism was diagnosed in about 0.15 per cent of the 62 865 CV samples, while confined placental mosaicism (CPM) occurred in 1.0 per cent. False‐positive non‐mosaic aberrations were observed in 0.15 per cent and false‐negative CVS (chorionic villus sampling) results in only 0.03 per cent. The remaining 0.15 per cent of the CVS results were unclassifiable. These figures determined a sensitivity of CVS for prenatal detection of chromosome aberrations of 98.9–99.6 per cent (95 per cent confidence intervals), a specificity of 98.5–98.8 per cent, a positive predictive value of 72.6–78.3 per cent, and a negative predictive value of 99.95–99.98 per cent. False‐positive non‐mosaic aberrations that could not from the outset be suspected of being confined to the placenta were very rare (0.07 per cent of CV samples). They most often involved non‐mosaic monosomy X and trisomy 18 encountered after direct preparation alone. False‐negative CVS results were extremely rare (0.03 per cent) and occurred, with only one exception, after direct preparation alone. Thirteen of the 19 false‐negative CVS diagnoses were from pregnancies at a particularly high risk for fetal chromosomal aberration. Seventy‐five per cent of the pregnancies with CVS mosaicism or non‐mosaic discrepancy and known outcome continued to livebirth. When CVS mosaicism was encountered, the definitive prenatal cytogenetic diagnosis was most often obtained through subsequent amniocentesis. However, the use of amniocentesis and the frequency of pregnancy termination depended on the type of chromosomal aberration involved. We conclude that CVS is an accurate method for prenatal chromosome analysis. In pregnancies at high risk for fetal chromosomal abnormality, we recommend, however, not relying solely on a normal karyotype obtained after direct preparation alone. © 1997 John Wiley & Sons, Ltd.

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