Background: Fetal crown-rump length (CRL) measurement by ultrasound in the first trimester is the standard method for pregnancy dating; however, a multitude of CRL equations to estimate gestational age (GA) are reported in the literature.

Objective: To evaluate the methodological quality used in studies reporting CRL equations to estimate GA using a set of predefined criteria.

Search strategy: Searches of MEDLINE, EMBASE, and CINAHL databases, from 1948 to 31 January 2011, and secondary reference sources, were performed.

Selection criteria: Observational ultrasound studies, where the primary aim was to create equations for GA estimation using a CRL measurement.

Data collection and analysis: Included studies were scored against predefined independently agreed methodological criteria: an overall quality score was calculated for each study.

Main results: The searches yielded 1142 citations. Two reviewers screened the papers and independently assessed the full-text versions of 29 eligible studies. The highest potential for bias was noted in inclusion and exclusion criteria, and in maternal demographic characteristics. No studies had systematic ultrasound quality-control measures. The four studies with the highest scores (lowest risk of bias) satisfied 18 or more of the 29 criteria; these showed lower variation in GA estimation than the remaining, lower-scoring studies. This was particularly evident at the extremes of GA.

Author's conclusions: Considerable methodological heterogeneity and limitations exist in studies reporting CRL equations for estimating GA, and these result in a wide range of estimated GAs for any given CRL; however, when studies with the highest methodological quality are used, this range is reduced.

BJOG: Napolitano R, Dhami J, Ohuma EO, Ioannou C, Conde-Agudelo A, Kennedy SH, Villar J, Papageorghiou AT. Pregnancy dating by fetal crown-rump length: a systematic review of charts. BJOG. 2014 Apr;121(5):556-65.