Crown–rump length measurement error: impact on assessment of growth

ABSTRACT 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. © 2021 International Society of Ultrasound in Obstetrics and Gynecology.