Optimal crown-rump length for measuring the nuchal translucency

PURPOSE: To determine the optimal crown-rump length (CRL) for obtaining nuchal translucency (NT) measurements. METHODS: Women undergoing NT measurements by NT-certified sonographers within a combined screening program over an 18-month period were included in the study if they had a living fetus with a CRL of 45-84 mm. NT measurement success and screen-positive rates, transvaginal sonography (TVUS) use, and reasons for failed NT measurements were compared in 3 groups by CRL corresponding to 11-0/7 to 11-6/7 weeks (45-54 mm), 12-0/7 to 12-6/7 weeks (55-66 mm), and 13-0/7 to 13-6/7 weeks (67-84 mm). RESULTS: Eight hundred thirty-seven women aged 34.9 +/- 4.9 years underwent 1 to 3 NT measurements at a mean CRL of 59.8 +/- 25.0 mm. NT measurements were more successful at 11 and 12 weeks (81.8% and 84.4%) than at 13 weeks (66.9%) (p < 0.001). Screen-positive and TVUS rates, and reasons for failed NT measurements did not vary by CRL. CONCLUSION: NT measurements are most successful at CRL 45-66 mm corresponding to sonographically determined gestational ages of 11-0/7 to 12-6/7 weeks.