Biparietal diameter vs crown–rump length as standard parameter for late first‐trimester pregnancy dating.

Objective: To compare the precision of biparietal diameter (BPD) and crown–rump length (CRL) as predictors of gestational age in the human fetus in the late first and early second trimesters, using a population‐based approach. Methods: We constructed term and gestational‐age prediction curves for first‐trimester dating, based on 11 041 pregnancies with 12 260 measurements of CRL and/or BPD from a population‐based Norwegian clinical database. We used a population‐based approach with local linear quantile regression, combined with a time‐to‐event strategy that compensates for induced births. Term prediction precision was assessed by estimating and comparing the prediction residual curves using a time‐to‐event analysis. Individual differences in gestational‐age predictions from CRL and BPD were assessed using measurements performed on the same fetus on the same day. A sensitivity analysis was performed to evaluate the effect of not distinguishing between non‐spontaneous and spontaneous births. Results: CRL and BPD provided almost identical term prediction precision judged from the residual distribution. In about 51% of examinations, the difference in predicted gestational age was 1 day or less; 24% of examinations had a difference of 2 days, 14% had a difference of 3 days, 7% had a difference of 4 days and only 5% of all examinations had a difference of 5 days or more. Incorrectly removing induced births from the analysis, or treating them as spontaneous, would cause a substantial systematic prediction bias of about 2 days. Conclusions: Based on population data, using comparisons at an individual level, our study found that BPD is as precise as CRL when used for first‐trimester dating. BPD has advantages from a clinical point of view, since it is technically less challenging and less time‐consuming to measure compared with CRL, and can be measured and assessed throughout the entire pregnancy. © 2024 The Author(s). Ultrasound in Obstetrics & Gynecology published by John Wiley & Sons Ltd on behalf of International Society of Ultrasound in Obstetrics and Gynecology. [ABSTRACT FROM AUTHOR]