Two-stage approach for prediction of small-for-gestational-age neonate and adverse perinatal outcome by routine ultrasound examination at 35-37 weeks' gestation

Background: Justification of prenatal screening for small for gestational age (SGA) fetuses near term is based on first, evidence that such fetuses / neonates are at increased risk of stillbirth and adverse perinatal outcome, and second, the expectation that these risks can be reduced by medical interventions, such as early delivery. However, there are no randomized studies demonstrating that routine screening for SGA fetuses and appropriate interventions in the high risk group can reduce adverse perinatal outcome. Before such meaningful studies can be undertaken it is essential that first, the best approach for effective identification of SGA neonates is determined, and second, the contribution of SGA neonates to the overall rate of adverse perinatal outcome is established. In a previous study of pregnancies that had undergone routine ultrasound examination at 35+0 36+6 weeks’ gestation, we found that first, screening by estimated fetal weight (EFW) <10th percentile provided poor prediction of SGA neonates and second, prediction of >85% of SGA neonates requires use of EFW <40th percentile.

Objectives: First, to examine the contribution of SGA fetuses to the overall rate of adverse perinatal outcome and second, to propose a two stage approach for prediction of SGA neonates at routine ultrasound examination at 35+0 36+6 weeks’ gestation.

Methods: This was a prospective study of 45,847 singleton pregnancies that had undergone routine ultrasound examination at 35+0 36+6 weeks’ gestation. First we examined the relationship between birthweight percentile and adverse perinatal outcome, defined as stillbirth, neonatal death or admission to the neonatal unit for ≥48 hours. Second, we used a two stage approach for prediction of SGA neonates and adverse perinatal outcome; in the first stage fetal biometry was used to distinguish pregnancies at very low risk (EFW ≥40th percentile) and those at increased risk (EFW <40th percentile) and in the second stage the pregnancies with EFW <40th percentile were stratified into high--, intermediate and low risk groups based on the results of EFW and pulsatility index (PI) in the uterine arteries (UtA PI), umbilical artery (UA PI) and fetal middle cerebral artery (MCA PI). Different percentiles in EFW and Doppler indices were used to define each risk category and the performance of screening for SGA neonates and adverse perinatal outcome in babies born at ≤2, 2.1 4 and >4 weeks after assessment was determined. We propose that the high risk group would require monitoring from initial assessment to delivery, the intermediate risk group would require monitoring from two weeks after initial assessment to delivery, the low risk group would require monitoring from four weeks after initial assessment to delivery, and the very low risk group would not require any further reassessment.

Results: First, although in babies with low birthweight (<10th percentile) the risk of adverse perinatal outcome is increased, 84% of adverse perinatal events occur in the group with birthweight ≥10th percentile. Second, in screening by EFW <10th percentile the predictive performance for SGA neonates is modest for those born at ≤2 weeks of assessment (83% and 69% for neonates with birthweight <3rd and <10th percentiles, respectively), but poor for those born at 2.1 4 weeks (61% and 45%) and >4 (40% and 30%) from assessment. Third, improved performance of screening, especially for those delivering after two weeks from assessment, is potentially achieved by a proposed new approach for stratifying pregnancies into management groups based on findings of EFW and Doppler indices (prediction of birthweight <3rd and <10th percentiles for deliveries at ≤ 2, 2.1 4 and >4 weeks from assessment: 89% and 75%, 83% and 74% and 88% and 82%, respectively). Fourth, the predictive performance for adverse perinatal outcome of EFW <10th percentile is very poor (26%, 9% and 5% for deliveries at ≤ 2, 2.1 4 and >4 weeks from assessment, respectively) and this is improved by the proposed new approach (31%, 22% and 29%).

Conclusion: The study presents an approach for stratifying the pregnancies undergoing routine ultrasound examination at 35+0 36+6 weeks’ gestation into four management groups based on findings of EFW and Doppler indices. This approach can potentially have a higher predictive performance for SGA neonates and adverse perinatal outcome than screening by EFW <10th percentile.

Fetal Medicine Foundation Grant Number: 1037116