(594) Does Adding Abdominal Circumference to Estimated Fetal Weight Predict Neonatal Morbidity in Growth Restriction?

Objective: SMFM and ACOG diagnostic criteria for fetal growth restriction (FGR) recently added abdominal circumference (AC) < 10th %ile as an additional criterion to traditional estimated fetal weight (EFW) < 10th %ile; however, how well this addition predicts neonatal morbidity (NNM) is unknown. We compared AC or EFW (EFW+AC) to EFW alone in the prediction of composite NNM (CNNM) and small-for-gestational-age (SGA) at birth.

Study Design: Secondary analysis of the NICHD Fetal Growth Study-Singletons with research quality ultrasounds from 2,802 low-risk patients at 12 US sites, 2009-2013. The last 3rd trimester ultrasound scan (mean gestational age 36.6 +/- 2.3 wks) was included from non-anomalous fetuses with known neonatal outcomes. Both EFW & AC were evaluated using NICHD charts. Outcomes were: 1) CNNM (see Figure footnote), and 2) SGA, defined as birthweight < 10th %ile by Fenton charts. Logistic regression assessed association of each FGR definition with outcomes and receiver operator characteristic area under the curve (AUC) assessed comparative predictive ability.

Results: Of the 2,397 patients included, 7.4% were SGA at birth and 5.6% of neonates had CNNM; 6.3% of patients met criteria for FGR by EFW+AC vs 5.1% by EFW alone (p< 0.0001). Although both FGR definitions had acceptable discrimination of SGA- EFW+AC: AUC 0.72 [95% CI, 0.68-0.75], EFW alone: AUC 0.69 [95% CI, 0.66-0.73], risk of CNNM was not associated with either definition- EFW+AC: RR 0.75 [95% CI, 0.42-1.4], EFW alone: RR 0.60 [95%CI 0.33-1.1], and both were poor predictors of CNNM- EFW+AC: AUC 0.51 [95% CI, 0.49-0.53], EFW alone: AUC 0.52 [95% CI, 0.49-0.54] (Figure).

Conclusion: The addition of AC < 10th %ile to the traditional definition of FGR significantly increased the incidence of FGR, but it did not improve the prediction of CNNM compared to EFW alone in a low-risk population. Given the use of prenatal resources and risks associated with the diagnosis of FGR, future studies should evaluate whether the new definition is superior in prediction of NNM and so confirm its clinical utility.