Objective: To examine the impact of first-trimester crown–rump length (CRL) measurement error on the interpretation of estimated fetal weight (EFW) and classification of fetuses as small-, large- or appropriate-for-gestational age on subsequent growth scans. Methods: We examined the effects of errors of ± 2, ± 3 and ± 4 mm in the measurement of fetal CRL on percentiles of EFW at 20, 32 and 36 weeks' gestation and classification as small-, large- or appropriate-for-gestational age. Published data on CRL measurement error were used to determine variation present in practice. Results: A measurement error of −2 mm in first-trimester CRL shifts an EFW on the 10^th percentile at the 20-week scan to around the 20^th percentile, and the effect of a CRL measurement error of + 2 mm would shift an EFW on the 10^th percentile to around the 5^th percentile. At 32 weeks, a first-trimester CRL measurement error would shift an EFW on the 10^th percentile to the 7^th (+ 2 mm) or 14^th (−2 mm) percentile; at 36 weeks, the EFW would shift from the 10^th percentile to the 8^th (+ 2 mm) or 12^th (−2 mm) percentile. Published data suggest that measurement errors of 2 mm or more are common in practice. Conclusion: Because of the widespread and potentially severe consequences of CRL measurement errors as small as 2 mm on clinical assessment, patient management and research results, there is a need to increase awareness of the impact of CRL measurement error and to reduce measurement error variation through standardization and quality control.