Background: The sonographic assessment of estimated fetal weight (EFW) is essential for identification of fetuses in weight extremes and aids in peripartum management. However, there are inconsistent reports regarding EFW accuracy.

Objective: To examine maternal and fetal determinants associated with unreliable EFW.

Methods: A retrospective case-control study was conducted at a single, tertiary medical center between 2011 and 2019. All term, singleton deliveries with a sonographic EFW within 2 weeks of delivery were included. Unreliable EFW was defined as > 500 grams discordance between it and the actual birth weight. We allocated the study cohort into two groups: unreliable EFW (cases) and accurate EFW (controls).

Results: Overall, 41,261 deliveries met inclusion criteria. Of these, 1721 (4.17%) had unreliable EFW. The factors positively associated with unreliable EFW included body mass index > 30 kg/m², weight gain > 20 kg, higher amniotic fluid index, pregestational diabetes, gestational age > 41^0/7, and birth weight ≥ 4000 grams. On multiple regression analysis, pregestational diabetes (odds ratio [OR] 2.22, 95% confidence interval [95%CI] 1.56–3.17, P < 0.001) and a higher birth weight (OR 1.91, 95%CI 1.79–2.04, P < 0.001) were independently associated with unreliable EFW. On analysis of different weight categories, pregestational diabetes was associated with unreliable EFW only among birth weights ≥ 3500 grams (OR 3.28, 95%CI 1.98–5.44, P< 0.001) and ≥ 4000 grams (OR 4.27, 95%CI 2.31–7.90, P < 0.001).

Conclusion: Pregestational diabetes and increased birth weight are independent risk factors for unreliable EFW and should be considered when planning delivery management.