Reproducibility study of crown–rump length and biparietal diameter measurements in the first trimester

OBJECTIVE: This study aims to investigate the intra-observer and inter-observer variability of crown-rump length (CRL) and biparietal diameter (BPD) measurements in the first trimester. METHODS: A prospective observational study on 592 fetuses between 8 and 14 gestational weeks was conducted. Intra-class correlation coefficients (ICCs) were computed, and Bland-Altman analysis was carried out. RESULTS: The ICCs (95% confidence interval) and the 95% limits of agreement, expressed as a percentage of the average, for CRL's intra-observer and inter-observer variability were 0.979 (0.974-0.982, p < 0.001), +8.53% to -8.376%, and 0.968 (0.952-0.978, p < 0.001), +9.06% to -7.69%, respectively. The corresponding figures for BPD were 0.981 (0.977-0.984, p < 0.001), +7.41% to -7.51%, and 0.968 (0.952-0.978, p < 0.001), +6.65% to -7.25%. After conversion in days, the ICCs and 95% limits of agreement for CRL's intra-observer and inter-observer variability were 0.983 (0.980-0.986, p < 0.001), +2.88% to -2.84%, and 0.971 (0.957-0.980, p < 0.001), 2.83% to -2.60%, respectively. The corresponding figures for BPD were 0.982 (0.979-0.986, p < 0.001), +3.36% to -3.40%, and 0.968 (0.953-0.978, p < 0.001), +3.06% to -3.38%. CRL's standard deviation of the differences increased with gestation for intra-observer (r = 0.289, p < 0.001) and inter-observer (r = 0.197, p = 0.023) variability. CONCLUSION: The BPD and CRL are highly reproducible measurements. CRL's measurement error increased with the magnitude of CRL, whereas BPD's reproducibility was not affected by gestational age.