Fetal growth restriction (FGR), formerly known as intrauterine growth restriction, is a known cause of neonatal morbidity. The goal of diagnosis of FGR is to prevent stillbirth and adverse neonatal outcomes, while attempting to prevent unnecessary interventions. The Society for Maternal-Fetal Medicine (SMFM) Consult Series #52: Diagnosis and management of fetal growth restriction, published in 2020, recommends the use of abdominal circumference (AC) or estimated fetal weight (EFW) less than the 10th percentile to diagnose FGR, as opposed to only EFW less than the 10th percentile. This study compares 3 methods of FGR diagnosis.

This retrospective cohort study classified three groups of women, delivering at our institution over three one-year periods, based on FGR diagnosis method used during that time: Group 1 (7/1/17-6/30/18) EFW < 10%ile, Group 2 (4/1/20-3/31/21) EFW < 10%ile or EFW 10-19%ile and AC < 5%ile, and Group 3 (8/1/21-7/31/22) EFW and/or AC < 10%ile. Multifetal gestations and fetuses with major anomalies were excluded. Cases of neonatal small for gestational age (SGA) were also identified in each time period. The primary outcome was prevalence of FGR among all deliveries in each time period. Test performance characteristics of the three methods of FGR diagnosis for predicting SGA were evaluated.

320 pregnancies with FGR were identified (n=44 Group 1, n=96 Group 2, n=180 Group 3). Maternal characteristics were similar between groups. For groups 1, 2 and 3 respectively, the prevalence of FGR was 1.16%, 2.74% and 4.82% (p< 0.0001), and the prevalence of SGA was 3.39%, 4.44% and 4.53% (p=0.02). The sensitivity of FGR diagnosis in detecting SGA was 25.8%, 35.5%, and 42.6% for Groups 1-3, respectively. The positive predictive value (PPV) of FGR diagnosis for SGA was 75.0%, 57.3%, and 40.0% for Groups 1-3, respectively.

Application of the newest diagnostic criteria for FGR in our delivery population resulted in a quadrupling of the rate of FGR diagnosis, with higher sensitivity, lower PPV, and unchanged NPV compared with the older definition. Though the prevalence of FGR diagnosis with the newest criteria was similar to the prevalence of SGA, this approach identified less than half of neonates with SGA. Given this substantial increase in diagnoses, most of which are false positives, further investigation regarding resource utilization and neonatal outcomes is warranted.