The Performance of Selective Screening Ultrasound to Detect the Small-for-Gestational-Age Foetus: A Prospective Cohort Study Nested Within the DESiGN Randomised Control Trial

Objective:
Evidence on the performance of foetal growth ultrasound parameters to detect the Small-for-Gestational-Age (SGA) foetus for a selective cohort is limited. We report the performance of estimated foetal weight under the 10th percentile (EFW < 10th) at the last selective screening ultrasound to detect SGA based on gestational age of scan and scan to birth interval.

Design:
We performed a prospective cohort study to analyse ultrasound data obtained from the DESiGN trial.

Setting:
The DESiGN trial was the first cluster randomised controlled trial to evaluate the impact of the Growth Assessment Protocol (GAP) on antenatal detection of SGA.

Population:
Ninety-four thousand, nine hundred twenty-five women included in DESiGN who had at least 1 ultrasound scan after 24 weeks' gestational age with complete ultrasound data underwent analysis.

Methods:
Test performance characteristics of EFW (by Hadlock 1991) < 10th percentile to predict birthweight < 10th percentile (by 1990 UK Population Charts) were reported at different gestational ages of scan categories and scan to birth intervals.

Results:
The performance of EFW < 10th percentile at the last selective screening ultrasound to correctly identify SGA declined from 34 weeks and with increasing scan to birth intervals. A scan between 34 and 37 + 6 weeks' gestation more than 4 weeks prior to birth had a particularly poor performance (sensitivity 4.0%, 95% CI 3.8–4.1).

Conclusions:
In a selected population, the performance of late third-trimester ultrasound estimation of foetal size to detect SGA at birth is poor.