Criteria for fetal nuchal thickness cut-off: a re-evaluation.

An attempt has been made to establish a more effective cut-off criterion for nuchal thickness (NT) and to assess the optimal gestational period for the prediction of trisomies 21 and 18. Reference intervals were established for NT from the tenth to the 18th week, using either gestation-specific centiles or the parametric method. The measurements in 47 consecutive trisomy 21 and 18 trisomy 18 cases were plotted against these intervals. Assaying different cut-off criteria for both the centile and the parametric methods, sensitivities and false-positive rates for each gestational week were calculated and then compared with the commonly applied 'two-stepped' cut-off method (3 mm early, 6 mm later). The parametric method, based on a progressive rise, with +2.5 SD for the corresponding gestational week as a cut-off value, showed the best performance (likelihood ratio 38) in the prediction for trisomy 21. The optimal gestational age was the 12-18 week period, with an overall sensitivity of 62 per cent (23/37) for an average false-positive rate of 0.7 per cent. For trisomy 18, the most effective cut-off was also +2.5 SD, and 10-13 gestational weeks as the optimal period, achieving 86 per cent (6/7) sensitivity for a 1.9 per cent false-positive rate.