TOPLINE:

Ultrasound-based foetal weight estimation accuracy varied significantly by foetal characteristics; foetuses estimated to be severe small for gestational age (SGA) had the poorest accuracy, which was even more pronounced when the ultrasound examination was performed at an earlier GA.

METHODOLOGY:

• Researchers conducted a prospective cohort study to evaluate the effect of maternal and foetal characteristics on the accuracy and precision of sonographic foetal weight estimation.
• They evaluated 31,521 singleton pregnancies recorded in the Swedish Pregnancy Register from 2014 to 2021 resulting in delivery at ≥ 22 + 0 weeks of gestation; all participants had an ultrasound-based estimated foetal weight (EFW) assessment within 2 days before delivery.
• EFW was calculated according to the formula of Persson and Weldner using foetal biometric ultrasound measurements. After standardisation for GA, each measurement of EFW was classified as severe SGA, mild SGA, appropriate for GA (AGA), mild large for GA (LGA), or severe LGA.
• Poor EFW assessment was defined as EFW deviating more than ±10% from the birth weight.

TAKEAWAY:

• The risk for poor FW estimation was higher in foetuses estimated as severe SGA than in those estimated as AGA (adjusted risk ratio [aRR], 1.44; 95% CI, 1.38-1.51).
• Male vs female foetuses (aRR, 1.09; 95% CI, 1.05-1.14) and those with non-cephalic vs cephalic presentation (aRR, 1.20; 95% CI, 1.13-1.28) had a higher risk for poor FW estimation.
• Furthermore, the risk for poor FW estimation was higher in foetuses with a GA at ultrasound of 22 + 0 to 27 + 6 weeks (aRR, 1.30; 95% CI, 1.13-1.50), 28 + 0 to 31 + 6 weeks (aRR, 1.26; 95% CI, 1.13-1.40), and 32 + 0 to 36 + 6 weeks (aRR, 1.17; 95% CI, 1.10-1.24) than in those with a GA at ultrasound of 39 + 0 to 40 + 6 weeks.

IN PRACTICE:

"It is essential to keep in mind the uncertainty and potential error of fetal weight estimation when basing clinical decisions on EFW. Knowledge of the weaknesses of the chosen formula, especially in the extremes of standardized EFW, is of great importance in clinical decision-making," the authors wrote.

SOURCE:

This study was led by Michaela Granfors, Division of Clinical Epidemiology, Department of Medicine (Solna), Karolinska Institutet, Stockholm, Sweden. It was published online on October 24, 2025, in Ultrasound in Obstetrics & Gynecology.

LIMITATIONS:

Pregnancies at a high risk for abnormal foetal growth were overrepresented, which may have limited the generalisability of the findings. This study lacked detailed information on the competence of the ultrasound examiner. Foetal presentation was recorded only at birth rather than at the time of ultrasound examination, which may have led to misclassification.

DISCLOSURES:

This study was funded by the Swedish Infant Death Foundation. No information regarding conflicts of interest was provided for this study.

This article was created using several editorial tools, including AI, as part of the process. Human editors reviewed this content before publication.