AbstractPurpose.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. © 2007 Wiley Periodicals, Inc. J Clin Ultrasound, 2007