Introduction Ultrasound estimated fetal weight is increasingly being used in the monitoring of fetal growth. Differences between estimated fetal weight formulae, curves and measurement methods could lead to significant differences in results. The aim of this study was to investigate the potential impact of these differences on estimated fetal weight and its use in monitoring fetal growth, both by modelling and by analysis of ultrasound scan data. Methods Four estimated fetal weight curves were compared in their original form and also normalised to term weight. Estimated fetal weight was calculated from 50th centiles of widely used charts of abdominal and head circumference and femur length and plotted on a widely used estimated fetal weight curve. Fetal measurement data were used to assess the impact of fetal proportions on estimated fetal weight error and on growth trajectory when different estimated fetal weight formulae are used. Results Estimated fetal weight curves differ significantly, but after normalisation there is closer agreement. Estimated fetal weight modelled using modern measurement methods differs from the widely used estimated fetal weight growth curve. Errors in estimated fetal weight are correlated with differences in fetal proportions and this can lead to significant changes in estimated fetal weight growth trajectory if different estimated fetal weight formulae are used. Conclusions Choice of measurement methods, estimated fetal weight formulae and growth curves have a significant effect on estimated fetal weight growth trajectories relative to normal ranges. It is important to understand these caveats when using estimated fetal weight to monitor fetal growth.

Introduction: Ultrasound estimated fetal weight is increasingly being used in the monitoring of fetal growth. Large systematic and random errors in estimated fetal weight have been reported; these may have an impact on the accuracy of fetal growth monitoring. The aim of this study was to attempt to evaluate these systematic and random errors by analysis of serial ultrasound data. Methods: Ultrasound measurements and birthweights were retrospectively collected for 100 unselected patients who had undergone serial ultrasound. Birthweights were used to calculate expected fetal growth trajectories using a method for generating growth charts based on customised birthweights. Estimated fetal weight results were then compared with the expected growth trajectories to evaluate systematic and random differences. Results: Incomplete measurement sets were excluded, reducing the number of scans to less than three for 13 subjects. A further 17 subjects with suspected pathological growth trajectories were excluded. The final analysis included 70 subjects with a total of 246 scans. The mean difference between estimated fetal weight and expected weight over three to six scans ranged from −17.5% to 38.3% with a mean of 8.4%, representing the systematic difference. The standard deviation of these differences ranged from 0.4% to 21% with a mean of 4.3%, representing random difference. Conclusion: Systematic and random differences between estimated fetal weight and expected fetal weight are significant and make interpretation of fetal growth difficult. Further improvements to formulae and growth curves are required and audit of fetal measurements is essential to service improvement.