Accuracy of sonographic fetal biometry in estimating intertwin size discordance at birth.

Objective: To determine the accuracy of estimated fetal weight (EFW) and abdominal circumference (AC) discordance in diagnosing large birthweight (BW) discordance and selective fetal growth restriction (sFGR).

Methods: Retrospective cohort study of patients with twin pregnancies followed at a tertiary center (N = 1065). We determined the accuracy of intertwin fetal size discordance (based on either EFW or fetal AC) at the last ultrasound exam before birth in estimating birthweight discordance and in diagnosing sFGR at birth.

Results: EFW discordance was more accurate than AC discordance in estimating BW discordance as reflected by a smaller systematic error (-0.97% vs. -6.43%, respectively, P < 0.001) and mean absolute percentage error (5.70% vs. 7.35%, respectively, P < 0.001), and a larger proportion of cases with discordance within 5%, 10%, or 15% of BW discordance (53.1% vs. 45.4%, 84.3% vs. 72.4%, and 95.1% vs. 87.0%, respectively). Still, both EFW discordance and AC discordance had low diagnostic accuracy for large BW discordance and sFGR. For example, EFW discordance >20% had a sensitivity of 49.1% and a positive predictive value of 57.0% for BW 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.

Conclusion: While EFW discordance was more accurate than AC discordance in estimating BW discordance, both measures had low diagnostic accuracy for large BW discordance and sFGR. Care providers should consider the limited diagnostic accuracy when making management decisions on the timing and mode of delivery.