THE WEIGHT OF EXPECTATION: A CRITICAL LOOK AT ULTRASOUND ACCURACY IN SUSPECTED LGA PREGNANCIES
Delillah Muzulu, Catriona Hynes, Sheffield Hallam University
Introduction:
Accurate fetal weight estimation is vital in antenatal care, since ultrasound-estimated fetal weight (EFW) guides timing and mode of delivery, particularly in cases of fetal growth restriction (FGR), macrosomia, or large-for-gestational-age (LGA). Ultrasound accuracy in predicting macrosomia and LGA remains uncertain, and inaccurate EFW can lead to poor clinical decisions and adverse outcomes. This study aimed to assess the accuracy of ultrasound EFW in predicting birth weight and explored influencing factors.
Methods:
A retrospective cohort study of 239 singleton pregnancies delivered at a single NHS Trust between January 2023 and June 2024 was conducted. Third-trimester ultrasounds (>35 weeks) for suspected macrosomia or LGA were included where delivery occurred within 14 days. EFW was calculated using the Hadlock formula and plotted on GROW 2.0 customised growth charts.
Accuracy was evaluated using absolute error (AE) and percentage error (PE), with PE classified as positive (overestimation) or negative (underestimation). EFW was considered accurate if within ±15% of birth weight. Inferential statistical analysis was performed using SPSS v26.0.1.1.
Results:
Ultrasound EFW underestimated birth weight by a mean of 68g (p = 0.001). A correlation coefficient and regression beta of 0.682 indicated a moderate-to-strong positive relationship between EFW and actual birth weight. Bland–Altman analysis revealed wide variability, with limits of agreement from -672g to +535.8g,
showing notable disagreement.
EFW was within ±10% of actual birth weight in 133 cases (76.6%), 10.01–15% in 44 cases (18.4%) and exceeded 15% in 11 cases (4.6%). Accuracy was affected by scan-to-delivery interval and gestational age at the time of ultrasound.
Conclusion:
Ultrasound EFW moderately predicts macrosomia and LGA, but often underestimates birth weight; therefore, delivery decisions should carefully interpret ultrasound findings. Broader multicentre studies, including normal EFW cases, are required to improve evidence accuracy.