Optimized Sonographic Weight Estimation of Fetuses over 3500 g Using Biometry-Guided Formula Selection

Area of Science:

• Maternal-Fetal Medicine
• Medical Imaging
• Biometry

Background:

• Existing fetal weight estimation formulas like Hadlock et al. show underestimation in larger fetuses (>3500g).
• The Merz et al. formula is more accurate for larger fetuses but less so for smaller ones.
• A need exists for improved fetal weight estimation accuracy across the full range of fetal sizes.

Purpose of the Study:

• To enhance fetal weight estimation accuracy, particularly for fetuses exceeding 3500g.
• To identify the optimal fetal biometric parameter for guiding the selection of appropriate weight estimation formulas.
• To develop and validate a new method for improved fetal weight prediction.

Main Methods:

• Regression analysis of 12,032 pregnancies identified abdominal circumference × femur length (AC × FL) as a key parameter.
• A threshold for AC × FL (24,600) was established to switch between the Hadlock (below threshold) and Merz (at or above threshold) formulas.
• This 'Zurich method' was prospectively validated in 4073 pregnancies, comparing its accuracy (Percentage Error, Absolute Percentage Error) against Hadlock, Merz, and Kehl formulas across various birth weight categories.

Main Results:

• The Zurich method demonstrated lower Percentage Error (PE) for fetuses >3500g compared to Hadlock alone.
• For fetuses <1500g, the Zurich method achieved the lowest PE, comparable to the Hadlock formula.
• In terms of Absolute Percentage Error (APE), the Zurich method performed comparably to the Merz formula for fetuses >4000g and matched the Hadlock formula for fetuses <1500g.

Conclusions:

• The Zurich method effectively utilizes a pivotal value of AC × FL to dynamically select between established formulas.
• This approach corrects the underestimation errors of the Hadlock formula in fetuses ≥3500g.
• The Zurich method also mitigates the inaccuracies of the Merz formula in fetuses <3500g, offering improved overall fetal weight estimation.