Ultrasonographic estimation of fetal weight: development of new model and assessment of performance of previous models

Objective: To develop a new formula for ultrasonographic estimation of fetal weight (EFW) and evaluate the accuracy of this and all previous formulas in the prediction of birthweight.

Methods: The study population consisted of 5,163 singleton pregnancies with fetal biometry at 22-43 weeks’ gestation and livebirth of phenotypically normal neonates within 2 days of the ultrasound examination. Multivariable fractional polynomial analysis was used to determine the combination of variables that provided the best-fitting models for EFW. A systematic review was also carried out of articles reporting formulas for EFW and comparing EFW to actual birthweight.

The accuracy of each model for EFW was assessed by comparing the mean percentage error (MPE), absolute mean error (AE), proportion of pregnancies with an AE <10% and Euclidean
distance.

Results: The most accurate models, with the lowest Euclidean distance and highest AE <10%, were provided by the formulas incorporating >3 rather than <3 biometrical measurements. The systematic review identified 45 studies describing a total of 70 models for EFW by various combinations of measurements of fetal head circumference (HC), biparietal diameter (BPD), femur length (FL) and abdominal circumference (AC). The most accurate model with the lowest Euclidean distance and highest AE <10% was provided by the formula of Hadlock et al, published in 1985, which incorporated measurements of HC, AC and FL; there was a highly significant linear association between EFW with birthweight (r=0.959; p<0.0001) and the EFW was within 10% of birthweight in 80% of cases. The performance of the best model developed in this study, utilizing HC, AC and FL, was very similar to that of Hadlock et al.

Conclusion: Despite many efforts to develop new models for EFW, the one reported in 1985 by Hadlock et al,. from measurements of HC, AC and FL, provides the most accurate prediction of birthweight and can be used for assessment of all babies as well as those suspected to be either small or large.

Fetal Medicine Foundation. Grant Number: 1037116