Undermeasurement of Nuchal Translucencies: Implications for Screening

OBJECTIVE: To analyze the maximum nuchal translucency from 327 centers to determine whether a more-than-expected number of centers had maximum nuchal translucency of 2.5 mm or less (approximately 4% of nuchal translucency values should be 2.5 mm or higher). METHODS: We analyzed data from 182,669 nuchal translucency cases at centers in which at least 100 nuchal translucency examinations were performed from July 2008 through June 2009 and investigated the appropriateness of the distribution of values. We then investigated the likelihood of the skewing of the distribution seen using a 100 simulations of such modeled data. RESULTS: Based on a binomial distribution, the chance that a center would have no nuchal translucency values above 2.5 mm is 1.7% for 100 patients per center, and 0.2% for 150 patients per center. Additionally, the median multiples of the median should shift by approximately 2.5% if all nuchal translucency values higher than 2.5 mm are excluded from the population. Our data show that 7.3% of centers had a maximum nuchal translucency of to 2.5 mm or less, and more than 20% have never reported an nuchal translucency of greater than 3 mm. The maximum nuchal translucency at a center correlated positively with its median multiple of the median. Centers with no nuchal translucency values greater than 2.5 mm also have nearly 50% of their ultrasonographers with excessive low nuchal translucency (greater than 10% of cases less than fifth percentile). CONCLUSION: Too many centers have a maximum nuchal translucency of 2.5 mm or lower, low median nuchal translucency, and excessive low nuchal translucency, indicating that data from these centers are not representative of the expected distribution of nuchal translucencies. Our data suggest a systematic undermeasurement of nuchal translucency. LEVEL OF EVIDENCE: III