The role of sonographic estimated fetal weight compared to clinical assessment: superior accuracy only in extreme fetal weight cases

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BACKGROUND: Estimated fetal weight (EFW) is of key importance in antenatal care, as inaccuracies can contribute to adverse outcomes and unnecessary interventions. However, both clinical and sonographic methods for EFW have demonstrated limited accuracy, and different factors were found to be associated with increased error range. This study aimed to compare the accuracy of sonographic and clinical EFW methods and identify factors influencing estimation errors. METHODS: This retrospective analysis included singleton pregnancies delivering at ≥ 37 weeks of gestational age between 2014 and 2023. Participants with recorded clinical and sonographic EFW within 14 days of delivery were included. Exclusion criteria included multiple gestations, absence of either clinical or sonographic EFW, and incomplete data. EFW accuracy was evaluated against actual neonatal birthweight using overall absolute error and absolute percent error, using paired t-test. Differences in error categories (> 10% and > 20%), specificity, sensitivity, positive predictive value (PPV) and negative predictive value (NPV) were evaluated using McNemar test. Subgroup analyses included normal weight ( (BMI 18.5–24.9), obese patients (BMI ≥ 30), tall women (height > 172 cm), nulliparous and multiparous women, and neonates with birth weights ≥ 4500 g, 4000–4499 g, and ≤ 2500 g. Logistic regression identified factors associated with > 10% error in each method. RESULTS: Out of 61,531 term deliveries in the study period,14,400 women were included in the study. Sonographic EFW demonstrated significantly higher accuracy among birthweights extremes of ≤ 2500 g and ≥ 4500 g with lower mean absolute percent error in these groups (11.3% vs. 15.0% and 10.6% vs. 12.3%, respectively). Clinical estimation showed a higher negative predictive value for ruling out extreme birthweight (54.9% vs. 44.1%), supporting its usefulness as a first-line screening tool in routine obstetric care. While statistically significant differences were observed in favor of sonographic EFW across other subgroups and the entire study population, no differences of clear clinical relevance were found, as absolute differences were minimal. Low birth weight, macrosomia ≥ 4500 g and nulliparity were associated with a high estimation error in both sonographic and clinical EFW, with a greater extent observed in clinical EFW. CONCLUSIONS: Sonographic EFW exhibits superior accuracy in cases of extreme birth weights (≤ 2500 g and ≥ 4500 g). Among other populations, differences between sonographic and clinical EFW were minimal and of limited clinical relevance. This knowledge should be incorporated into daily practice and decision-making processes to ensure optimal patient care.

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