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Correct estimation of gestational age is essential for any study of ultrasound biometry and for everyday clinical practice. However, inconsistency in pregnancy dating may occur through differences in measurement methods or errors during measurement. This paper describes the standardised crown-rump length (CRL) measurement methodology and interpretation used across study sites in the INTERGROWTH-21st Project. These methods should minimise potential systematic errors in dating associated with pooling data drom different health institutions, and represent a model for standardising CRL measurement in future studies.