Accuracy and modifying factors of the sonographic estimation of fetal weight in a high-risk population.

Abstract

There have been a number of reviews assessing the accuracy of different methods of sonographic estimation of fetal weight, without identifying any clearly superior equation. In order to optimise accuracy in a high-risk population, we decided to compare some of the most popular early equations with the newer volume-based equations, and to try and identify factors that affect the ability of these equations to estimate fetal weight accurately We collected the scan and delivery details of 192 fetuses born within one week of a sonographic estimation of fetal weight. We then applied three of the most popular equations and two newer volume-based equations to the recorded fetal biometric parameters to assess the performance of each equation overall, and under varying maternal, fetal, and scan conditions. The equations of Shepard, Hadlock A, Hadlock B and Combs produced similar results with systematic (mean) errors in the range 1.2-1.9% and random error characterised by one standard deviation in the range of 8.6-9.5%. Dudley's volume-based equation produced a significant systematic error in the form of a mean error of 7.4%, which corresponds to a mean birthweight (BW) which is 7.4% above the mean estimated fetal weight (EFW). When we stratified the study group by birthweight, Combs' equation produced significant differences in the mean error, (p < 0.00001), that ranged from a mean overestimation in fetal weight of 8.5% for babies with BW < 1000 g to a mean underestimation in fetal weight of 6.2% for babies with BW > 3000 g. Oligohydramnios resulted in a trend towards an increased mean error for all equations which was only statistically significant for Hadlock B. The equations Shepard and Hadlock A performed best in our high-risk population. They produced the smallest systematic errors across the entire study group and were not adversely affected by variations in birthweight, liquor volume, or fetal presentation. The newer, volume-based equations were disappointing, producing large systematic errors. Large random errors in all equations continue to be the Achilles' heel that limit the value of sonographic EFW.