Factors Associated with the Accuracy of Sonographic Fetal Weight Estimation: A Study of 1262 Vietnamese Pregnancies

Background/Objectives: Sonographic estimated fetal weight (EFW) guides many obstetric decisions. We examined maternal, fetal and sonographic factors associated with EFW error using the Hadlock II, Hadlock III and Shinozuka formulas. Methods: This is a retrospective cross-sectional study of 1262 singleton pregnancies at 30–41 weeks scanned within 3 days before delivery. Accuracy was assessed by mean percentage error (MPE), mean absolute percentage error (MAPE), successful estimation rate (SER; error ≤ 10%), Bland–Altman analysis with regression-based limits of agreement and Deming regression. Formulas were compared with paired tests. Each factor’s contribution was measured as the decrease in Nagelkerke R2 (ΔR2) in multivariable logistic regression; factor × formula interactions were tested with generalised estimating equations (GEE). Results: MAPE was 5.44%, 5.64% and 5.75% and SER 86.2%, 84.2% and 84.5% for Hadlock II, Hadlock III and Shinozuka, respectively; differences between formulas were small (ΔMAPE ≤ 0.31 percentage points). Against birth weight, all three overestimated small and underestimated large fetuses (Deming slopes 0.79–0.85). Birth weight was the strongest correlate of SER (decrease in Nagelkerke R2, 0.052–0.129) and, in the main analysis, the only factor whose effect differed between formulas (p < 0.001); other factors reduced Nagelkerke R2 by <0.02. Sensitivity was 42–66% for the 89 neonates < 2500 g and 18–38% for the 39 neonates ≥ 4000 g; positive predictive values were 78–82% and 88–100%, respectively, the latter based on 8–16 estimates (lower 95% confidence limits, 47–74%). Conclusions: EFW error was mainly associated with fetal size; this pattern, partly due to regression towards the mean, cannot be used as a bedside correction. Other characteristics showed little additional association with accuracy. The differences between the formulas were small, their clinical relevance is unclear, and none is recommended over the others. An EFW of 2500–2999 g does not exclude low birth weight, and an EFW of 3500–3999 g does not exclude a neonate of ≥4000 g.