Abstract

OBJECTIVE: Fetal growth assessment by ultrasound aims to identify small babies that are at higher risk of perinatal morbidity and mortality. The current study explores if the association between suboptimal fetal growth and adverse perinatal outcome varies with different definitions of fetal growth restriction and weight charts/standards. METHODS: This was a retrospective cohort study of 17261 singleton non-anomalous pregnancies from 24+0 weeks' gestation at a tertiary referral hospital. Estimated fetal weight (EFW) and Doppler indices were converted into gestational age specific centiles using a growth reference standard (Intergrowth-21) and various reference charts (Hadlock, Fetal Medicine Foundation [FMF] and Swedish). Test characteristics were assessed using definitions of FGR according to the International Society of Ultrasound in Obstetrics and Gynecology (ISUOG), Society of Maternal and Fetal Medicine (SMFM) and Swedish criteria. Adverse perinatal outcome was defined as perinatal death, admission to the neonatal intensive care (NICU) at term, 5' Apgar score < 7, and therapeutic cooling for neonatal encephalopathy. The association between FGR according to different definitions and adverse perinatal outcome was compared. Multivariate logistic regression was used to investigate the strength of the associations between ultrasound parameters and adverse perinatal outcome. Ultrasound parameters were also tested for correlation. RESULTS: Intergrowth-21 (IG-21), Hadlock and FMF fetal size references classified 1.47%, 3.55% and 4.5% fetuses respectively as FGR using the ISUOG definition and 2.87%, 8.82% and 10.6% fetuses respectively using the SMFM definition. The sensitivity of each of the definition/chart combinations for adverse perinatal outcome varied from 4.4% (ISUOG definition with IG-21 charts) to 13.2% (SMFM definition with FMF charts). The concomitant specificity also varied from 89.4% (SMFM definition with FMF charts) to 98.6% (ISUOG definition with IG-21 charts). ISUOG and Swedish criteria showed the highest specificity, positive predictive value, and positive likelihood ratio in detecting adverse outcomes irrespective of which fetal size reference charts/standards were used. Conversely, the SMFM definition had the highest sensitivity across all investigated growth charts. Low estimated fetal weight, elevated uterine artery mean PI, abnormal umbilical artery PI and abnormal cerebro-placental ratio were all significantly associated with adverse perinatal outcome and there was positive correlation between the covariates. Multivariate logistic regression showed that uterine artery Doppler mean PI and smallness (EFW below the 5th centile) were the only parameters to be consistently associated with adverse outcome irrespective of definitions or fetal size growth charts used. CONCLUSIONS: The prevalence of FGR is variable based on the specific definition as well as the fetal size reference chart used to diagnose FGR. Irrespective of the method of classification, the sensitivity for the identification of adverse perinatal outcome remains low. Estimated fetal weight, uterine artery and fetal Dopplers are all significant predictors of adverse perinatal outcome. As these indices are correlated to each other, a prediction algorithm is advocated to overcome the limitations of using them in isolation. This article is protected by copyright. All rights reserved.