COMPARISON OF THE HADLOCK AND INTERGROWTH-21^ST FORMULAE IN PREDICTING BIRTHWEIGHT CENTILES FOR LOW-RISK PREGNANCY; A SECONDARY ANALYSIS OF DATA

Objective: To compare the accuracy of the Hadlock to Intergrowth-21^st formulae in predicting birthweight for low-risk pregnancies. Methods: A secondary analysis of data from the Routine Third Trimester Ultrasound (ROTTUS) trial was undertaken for 284 low-risk pregnancies between 36 weeks and 37 weeks and 6 days gestation. The estimated fetal weight centiles using Hadlock, and Intergrowth-21^st formulae were computed, and the accuracy in predicting birthweight centile compared using ROC curve analysis and absolute percentage error. Logistic regression model was constructed based on ±15% as the dependent variable, to determine factors affecting the accuracy of either method. Results: The proportion of fetuses within 15% of the birth weight was 78.2% and 62.7% (p<0.0001) for Hadlock and Intergrowth-21st methods, area under the curve =0.8209 (CI 0.74347-0.89838) and 0.872 (CI 0.80865-0.93544) respectively (p<0.039). The mean absolute percentage error was-7.2 and-10.1 (p<0.0001) for Hadlock and Intergrowth-21^st respectively. The accuracy of Intergrowth-21^st method was influenced by the ultrasound-to-delivery interval. Conclusion: Overall, the Hadlock formula was more accurate than Intergrowth-21^st in predicting birthweight centile for low-risk pregnancies, except in large for gestational age fetuses. Both methods demonstrated a low sensitivity and specificity.