Estimation of fetal weight using Hadlock's formulas: Is head circumference an essential parameter?

Area of Science:

• Obstetrics and Gynecology
• Medical Imaging
• Biostatistics

Background:

• Accurate fetal weight estimation is crucial for optimal obstetric management.
• Hadlock formulas are widely used for estimating fetal weight via ultrasound.
• Variations in these formulas exist, necessitating comparison of their clinical utility.

Purpose of the Study:

• To compare the accuracy of two Hadlock fetal weight estimation formulas: one including head circumference (H1) and another excluding it (H2).
• To identify patient-specific factors influencing the reliability of the H2 formula.

Main Methods:

• Retrospective cohort study of 1220 sonographic fetal weight estimations within seven days of delivery.
• Comparison of H1 and H2 formula accuracy using percentage error, ±15% error interval, and Bland-Altman analysis.
• Multivariate regression analysis to identify factors affecting H2 formula accuracy.

Main Results:

• The H2 formula showed a significantly higher mean percent error (3.19%) compared to H1 (1.87%).
• H1 demonstrated higher accuracy in predicting fetal weight within ±15% of birth weight (93.44% vs. 90.49% for H2).
• Long femur length and low maternal age were significant factors affecting H2 formula reliability.

Conclusions:

• The Hadlock formula including head circumference (H1) is more accurate for term fetal weight estimation than the formula excluding it (H2).
• The accuracy difference is small, suggesting H2 can be used when head circumference measurement is challenging.
• Clinical caution is advised with H2 in cases of long femur length and lower maternal age.