Estimation of fetal weight: accuracy of regression models versus accuracy of ultrasound data

OBJECTIVES: To determine the respective importance of equation accuracy and variability in interexaminer measurements in estimation of fetal weight (EFW). METHODS: The study included 3 samples from three different French maternity hospitals. Sample 1 (6,508 fetuses) was used to compute a new linear regression model estimating fetal weight (FW) from ultrasound measurements. Sample 2, with 705 fetuses, was used to compare the accuracy of the new equation with Hadlock's equation. Sample 3 (1,461 fetuses) was used to assess - from our equation and from Hadlock's - the estimation errors due to the variability of ultrasound measurements recorded by 11 distinct examiners. Accuracy was determined by the signed percent difference (%Diff). Statistical analysis included the F test for correlated variances and comparisons of correlated variances. RESULTS: The random error of our model is 6.8%, significantly lower (p < 0.01) than Hadlock's which is around 7.4% (8.9% for low FW and 7.4% for high FW). The variability of ultrasound measurements among the different examiners in sample 3 generated a significant variation (p < 0.01) in the random error, ranging from 6.7 to 12.5%. This significant variation was also observed using Hadlock's equation for the same sample (from 7.2 to 12%). CONCLUSION: The accuracy of EFW depends much more on the quality of ultrasound measurements than on the choice of equation. Nevertheless, optimizing the accuracy of EFW (about 6.7-6.8%) imperatively requires standardized data collecting