Prediction of small-for-gestational-age neonates at 35-37 weeks' gestation: contribution of maternal factors and growth velocity between 20 and 36 weeks

Objective: To evaluate the performance of ultrasonographic estimated fetal weight (EFW) at 35+0 - 36+6 weeks’ gestation in the prediction of small for gestational age (SGA) neonates and assess the additive value of first, maternal risk factors and second, fetal growth velocity between 20 and 36 weeks’ gestation in improving such prediction.

Methods: This was a prospective study of 44,043 singleton pregnancies that had undergone routine ultrasound examination at 19+0 - 23+6 and at 35+0 - 36+6 weeks’ gestation. Multivariable logistic regression analysis was used to determine whether addition of maternal risk factors and growth velocity, defined by a difference in EFW Z-scores or fetal abdominal circumference (AC) Z-scores between the third and second trimester scans divided by the time interval between them, improved the performance of EFW at 35+0 - 36+6 weeks in the prediction of delivery of SGA neonates with birthweight <10th and <3rd percentiles within two weeks and at any stage after assessment.

Results: Screening by EFW at 35+0 - 36+6 weeks’ gestation <10th percentile predicted 63.4% (95% CI 62.0, 64.7) of neonates with birthweight <10th percentile and 74.2% (95% CI 72.2, 76.1) of neonates with birthweight <3rd percentile born at any stage after assessment, at screen positive rate of 10%. The respective values for SGA neonates born within two weeks of assessment were 76.8% (95% CI 74.4, 79.0) and 81.3% (95% CI 78.2, 84.0). In the group of fetuses with EFW <10th percentile, 43.7% were born with birthweight ≥10th percentile. For a desired 90% detection rate of SGA neonates delivering at any stage after assessment the necessary screen positive rate would be 33.7% for SGA <10th percentile and 24.4% for SGA <3rd percentile. Multivariable logistic regression analysis demonstrated that in the prediction of SGA neonates with birthweight <10th and <3rd percentiles there was a significant contribution from EFW Z-score at 35+0 - 36+6 weeks’ gestation, maternal risk factors and AC growth velocity, but not EFW growth velocity. However, the area under the receiver operating characteristic curves for SGA neonates in screening by maternal risk factors and EFW Z-score was not improved by addition of AC growth velocity.

Conclusion: Screening for SGA neonates by EFW at 35+0 - 36+6 weeks’ gestation and use of a cut-off of the 10th percentile predicts 63% of affected neonates. Prediction of 90% of SGA neonates necessitates classification of about 35% of the population as being screen positive use of the 35th percentile cut-off in EFW. The predictive performance of EFW is not improved by addition of estimated growth velocity between the second and third trimesters of pregnancy.

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