The value of the measurement of nuchal translucency thickness for predicting fetal Down's syndrome and other aneuploidies was prospectively evaluated at 8-15 weeks of gestation in 1819 consecutive pregnancies scheduled for karyotyping by chorionic villus sampling. In 43 cases, a chromosomal unbalanced aberration was found. Two teams of ultrasonologists who examined patients attending either National Health Service (Series 1) or private practice clinics (Series 2) were involved in the study. The same type of ultrasound machine and standardized approach were used in both study groups. In those cases in which the maximum subcutaneous thickness of the translucency was 3 mm or greater, the incidence of chromosomal aberration was 18.6% compared to 1.7% in the cases in which this was below 3 mm. The sensitivity, specificity and relative risk for all aneuploidies were 30%, 96% and 10.83, respectively, and no difference was found between trisomy 21 and other types of aneuploidy. The sensitivity and specificity and relative risk were significantly higher at 9-10 weeks than between 11 and 15 weeks. The results were concordant in the two series; however, the overall values for sensitivity (20% vs. 39%), specificity (94% vs. 98%) and relative risk (4.13 vs. 24.20) were clearly higher in the group of private patients. The results obtained confirm the potential application of the measurement of nuchal translucency thickness for fetal aneuploidy screening before the end of the first trimester and suggest that a multiplicity of individual, structural and organizational factors may interact and play a crucial role in determining the actual efficiency of ultrasound screening programs.

With the exception of aspartate aminotransferase, all of the analyzed fetal variables were different from the maternal values, and most changed with gestational age. The mechanisms leading to these fetal specificities remain mostly uncertain, but the provision of reference ranges for several blood constituents may be useful in the differential diagnosis of fetal disease.

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