(703) Diagnostic Accuracy of Sonographic Fetal Weight Estimation for Intertwin Size Discordance

Friday, January 31, 2025

10:30 AM – 12:30 PM

Submitting Author and Presenting Author(s)

AV

Arietta Vayenas

Research Student
Sunnybrook Research Institute, Sunnybrook Health Sciences Centre
Toronto, Canada

Coauthor(s)

Sophia Rahimi, BSc, MSc

Temerty Faculty of Medicine, University of Toronto
Toronto, Oregon, Canada
NM

Nir Melamed, MD

Division of Maternal Fetal Medicine, Department of Obstetrics and Gynecology, Sunnybrook Health Sciences Centre
Toronto, Ontario, Canada

Objective: Antenatal diagnosis of intertwin size discordance can affect management decisions. Therefore, care providers need to be aware of the accuracy of antenatal ultrasound for size discordance at birth. The current study aimed to estimate the diagnostic accuracy of estimated fetal weight (EFW) and abdominal circumference (AC) for large birthweight discordance and the diagnosis of selective fetal growth restriction (sFGR) at birth.

Study Design: Retrospective cohort study of twin pregnancies followed at a single center (2011-2023) who had ultrasound within 14 days before birth. We compared the diagnostic accuracy of EFW discordance and AC discordance for birthweight discordance and the diagnosis of sFGR at birth.

Results: Of the 1,065 twin pregnancies included, the rate of birthweight discordance > 20% and > 25% was 15.5% and 6.7%, respectively. EFW discordance was more accurate than AC discordance in estimating birthweight discordance, as reflected by a lower systematic error and mean absolute percentage error (Table). Still, both EFW and AC discordance had low diagnostic accuracy for large birthweight discordance and sFGR. For example, EFW discordance >20% had a sensitivity of 49.1% and a positive predictive value of 57.0% for birthweight discordance >20%, and the antenatal diagnosis of sFGR had a sensitivity of 45.8-54.4% and a positive predictive value of 50.9-55.2% for the postnatal diagnosis of sFGR. A false positive diagnosis of large birthweight discordance was mainly the result of overestimation of EFW percentile in the larger twin (Figure A), while failure to detect large birthweight discordance was mainly the result of overestimation of EFW percentile in the smaller twin (Figure B).

Conclusion: EFW discordance is more accurate than AC discordance in estimating birthweight discordance. However, both antenatal measures have low diagnostic accuracy for large birthweight discordance and sFGR. Care providers should be aware of the limited diagnostic accuracy when making management decisions on the timing and mode of delivery.