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

OBJECTIVE

To assess the internal consistency of the Hadlock normal range for estimated fetal weight (EFW) and its clinical implications.

DESIGN

Methodological analysis and retrospective observational audit.

SETTING

Tertiary maternity hospital in Oxford, UK.

POPULATION: Women with a non-anomalous singleton pregnancy.

METHODS

Reanalysis of the original Hadlock reference EFW chart using functional data analysis (FDA) and penalised functional regression (PFFR-GAM), and systematic bias assessment across gestation. Retrospective EFW centile analysis in a population of pregnancies undergoing a growth scan between 35 + 0 and 36 + 6 weeks, calculating the EFW centile using both the published reference chart (chart-derived centile) and the reported regression equation (equation-derived centile).

OUTCOME MEASURES: Agreement between chart-derived and equation-derived centiles and the corresponding differences in classification of fetuses below the 3rd and 10th centiles.

RESULTS

There is a statistically significant disparity between the chart-derived and equation-derived centiles, most pronounced in the 3rd and 10th centiles where clinical implications are critical. The 3rd and 10th chart-derived centiles approximately correspond to the 1st and 6th equation-derived centiles. Conversely, the equation-derived 10th centile corresponds to the chart-derived 17th centile. In a population of 21 874 pregnancies undergoing a growth scan between 35 + 0 and 36 + 6 weeks, the chart classified 4.2% as below the 10th centile, compared with 7.2% using the equation, with an absolute difference of 3.0% (95% CI 2.8%-3.3%; McNemar's test; p < 0.001). Also, the chart classified 0.8% under the 3rd centile, compared with 2.3% using the equation, with an absolute difference of 1.5% (95% CI 1.3%-1.7%; McNemar's test; p < 0.001).

CONCLUSION

Systematic disparity exists between the published chart- and equation-derived centiles in the original Hadlock growth reference. The equation-derived centiles are closer to the expected nominal proportions of our population, whereas the chart-derived centiles significantly underestimate the incidence of small for gestational age.