New sonographic method for fetuses with a large abdominal circumference improves fetal weight estimation

Birth weight (BW) is an important prognostic parameter for neonatal morbidity and mortality. Commonly used weight formulas lack accuracy, especially at the lower and upper end of the fetal weight range. Fetal abdominal circumference (AC) as part of most of the commonly used equations has the greatest impact on weight estimation. It has been shown that formulas specifically designed for a small fetal AC can improve weight estimation. The aim was to find out whether a new formula specifically designed for fetuses with a large AC may also improve weight determination.

The study included 830 singleton pregnancies. The inclusion criteria were ultrasound examination with complete biometric parameters and an AC ≥ 36.0 cm within 7 days of delivery, and an absence of structural or chromosomal malformations. Two "best-fit" formulas were derived by forward regression analysis. The accuracy of the new formulas was compared with commonly used weight equations using percentage error (PE), absolute percentage error (APE), limits of agreement (LOA) and cumulative distribution.

New formula I had no systematic error while new formula II and the routine methods significantly overestimated fetal weight. The medians of the APE were the lowest among the new equations (5.77 and 7.25). The new formulas also demonstrated the narrowest LOA. Importantly, at all discrepancy levels (5 %, 10 %, 15 %, and 20 %), new formula I included significantly more cases than the commonly used methods.

These specifically designed equations help to improve fetal weight estimation for fetuses with an AC ≥ 36.0 cm. For optimal weight estimation, we recommend using new formula I.