Comparison of different sonographic methods to determine fetal abdominal circumference

In modern obstetric management, accurate estimation of fetal weight is an important prognostic parameter of neonatal morbidity and mortality, and a valuable tool for determining further treatment. Fetal abdominal measurements, mostly abdominal circumference (AC), are not only included in the majority of commonly used weight equations, but also have the greatest impact on weight estimation. This therefore underlines the importance of accurate determination of the AC. The aim of our study was to evaluate which method for AC calculation is the most accurate one and thereby gives the best results for fetal weight estimation.

This prospective study included 239 term pregnancies. Inclusion criteria were a singleton pregnancy and ultrasound examination with complete fetal biometric parameters within 3 days of delivery, and absence of structural or chromosomal malformations. For calculation of the sonographic AC, three different methods were used. As a reference method, AC of the infant was determined immediately after birth. The accuracy of fetal weight estimation was determined for all of these methods of AC measurement using a commonly used weight equation. Differences were compared by using percentage error (PE), absolute percentage error (APE), limits of agreement (LOA) and cumulative distribution.

The ellipse region-of-interest (ROI) method showed the best results of all sonographic methods. For estimating fetal weight, it had the lowest median APE of the sonographic methods and the lowest SD of the PE, as well as the narrowest LOA. Regarding the cumulative distribution, ellipse ROI technique included the most cases at all discrepancy levels (5, 10, 15, and 20%).

Accurate measurement of fetal AC is important for good fetal weight estimation. The differences between the three techniques were small; however, there was a significant advantage when using the ellipse ROI method.