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

Accurate estimation of fetal weight is a valuable tool for determining further obstetric management. Commonly used weight formulas lack accuracy, even though some equations appear to be favorable within defined weight ranges. However, due to the fact that fetal weight is not known in advance, it is not always clear which formula is suitable. In most of the commonly used equations, the fetal abdominal circumference (AC) is not only included but also has the greatest impact on weight estimation. The aim of our study was to develop and evaluate a new formula specifically designed for a small fetal AC in order to improve weight estimation.

The study included 323 pregnancies. The inclusion criteria were singleton pregnancy, ultrasound examination with complete biometric parameters and an AC ≤ 29.0 cm within 7 days of delivery, and an absence of structural or chromosomal malformations. Two "best-fit" formulas were derived by forward regression analysis. Finally, the accuracy of the new formulas was compared to commonly used weight equations by using the percentage error, absolute percentage error (APE), limits of agreement (LOA) and cumulative distribution.

Contrary to the routine methods, which significantly underestimated fetal weight, the new formulas did not have a systematic error. The medians of the APE were the lowest (7.13 and 7.16) when compared to other equations. Moreover, the new formulas demonstrated the narrowest LOA. At all discrepancy levels (5%, 10%, 15%, and 20%), the new formulas included significantly more cases than the commonly used methods.

The specifically designed equations help to improve fetal weight estimation for fetuses with an AC ≤ 29.0 cm. For optimal weight estimation, we recommend using the new formula II.