Abstract

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.