A Comparison of Methods for the Diagnosis of Fetal Growth Restriction Between the Royal College of Obstetricians and Gynaecologists and the American College of Obstetricians and Gynecologists.

OBJECTIVE: The Royal College of Obstetricians and Gynaecologists (RCOG) defines fetal growth restriction as ultrasound-estimated fetal weight less than the 10th percentile or abdominal circumference less than the 10th percentile; the American College of Obstetricians and Gynecologists (ACOG) defines fetal growth restriction as estimated fetal weight less than the 10th percentile alone. We compared each method's ability to predict small for gestational age (SGA) at birth.

METHODS: For this retrospective study of diagnostic accuracy, we reviewed deliveries at the University of New Mexico Hospital from January 1, 2013, to March 31, 2017. We included mothers with singleton, well-dated pregnancies and nonanomalous fetuses undergoing indicated fetal growth restriction surveillance with an ultrasound-estimated fetal weight within 30 days of delivery. Estimated fetal weights and percentiles were calculated using the Hadlock intrauterine growth curve. Small for gestational age was defined as birth weight less than the 10th percentile based on a recent, sex-specific curve. We calculated the area under the curve, sensitivity, specificity, and positive and negative likelihood ratios for various approaches using abdominal circumference and estimated fetal weight to diagnose fetal growth restriction, including the definitions endorsed by ACOG and RCOG.

RESULTS: We included 1,704 pregnancies with a mean ultrasonography-to-delivery interval of 14.0 days (±8.6). There were 235 SGA neonates (13.8%). The rate of fetal growth restriction was 13.6% when using ACOG's criteria and 16.9% according to RCOG's criteria (P=.007). The area under the curve of RCOG's diagnostic approach was 0.78 (95% CI 0.76-0.80), which was higher than ACOG's (0.76, 95% CI 0.74-0.78, P=.01). Sensitivities and specificities of the various methods were similar. Adopting estimated fetal weight or abdominal circumference less than the 10th percentile instead of estimated fetal weight alone to predict SGA at birth would correctly identify one additional case of SGA for each 14 patients assessed.

CONCLUSION: The diagnostic approach endorsed by RCOG is a marginally better predictor of SGA at birth compared with the method endorsed by ACOG. Future research should consider the potential benefits and harms of the different methods in different populations.