Internal Consistency of the Hadlock Estimated Fetal Weight Reference: Methodological Analysis

Area of Science:

• Obstetrics and Gynecology
• Fetal Medicine
• Biostatistics

Background:

• The Hadlock reference chart is widely used for estimating fetal weight (EFW).
• Internal consistency of the chart and its derived regression equation is crucial for accurate fetal growth assessment.
• Discrepancies can lead to misclassification of small for gestational age (SGA) fetuses.

Purpose of the Study:

• To evaluate the internal consistency of the Hadlock EFW normal range.
• To assess the clinical implications of any identified disparities between the chart and its regression equation.
• To determine if the Hadlock chart accurately reflects expected population proportions for fetal growth.

Main Methods:

• Methodological analysis and retrospective audit of pregnancies.
• Reanalysis of the original Hadlock EFW chart using functional data analysis (FDA) and penalized functional regression (PFFR-GAM).
• Retrospective analysis of EFW centiles in a cohort of 21,874 pregnancies between 35 and 36 weeks gestation, comparing chart-derived and equation-derived centiles.

Main Results:

• A statistically significant disparity was found between chart-derived and equation-derived EFW centiles.
• The chart-derived 3rd and 10th centiles corresponded to the equation-derived 1st and 6th centiles, respectively.
• The chart underestimated the incidence of SGA fetuses, classifying fewer infants below the 3rd and 10th centiles than the equation.

Conclusions:

• Systematic disparities exist between the published Hadlock chart and its regression equation.
• Equation-derived centiles more closely align with expected population proportions.
• The Hadlock chart significantly underestimates the incidence of small for gestational age fetuses, impacting clinical interpretation.