Grand Rounds

RCOG or ACOG for Predicting Small for Gestational Age Birth?

BACKGROUND AND PURPOSE:

RCOG and ACOG have differing definitions of fetal growth restriction
- RCOG: Estimated fetal weight <10th percentile or abdominal circumference <10th percentile
- ACOG: Estimated fetal weight <10th percentile
Blue et al. (Obstetrics & Gynecology, 2018) compared the efficacy of these two methods of predicting small for gestational age (SGA) birth

Retrospective Review (2013-2017)
Included mothers with singleton, well-dated pregnancies and noanomalous fetuses
- Indicated fetal growth restriction surveillance
- Ultrasound-estimated fetal weight within 30 days of delivery
- Hadlock intrauterine growth curve
Small for gestational age (SGA): Birth weight <10^th percentile based on a recent, sex-specific curve
Area under the curve (AUC), sensitivity, specificity, and positive and negative likelihood ratios were calculated
DeLong analysis used to determine significance, rather than overlapping CIs

1,704 pregnancies were included
- 235 (13.8%) of pregnancies results in SGA
Rate of fetal growth restriction differed between the two guidelines (P=.007)
- ACOG: 13.6%
- RCOG: 16.9%
RCOG’s diagnostic approach had a significantly higher area under the curve (P=.01)
- RCOG: 0.78 (95% CI 0.76–0.80)
- ACOG: 0.76 (95% CI 0.74–0.78)
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

The guidelines introduced by RCOG are better in predicting SGA, although not by a large margin
Benefit of better prediction is the identification of fetuses at risk for stillbirth
This study does not address outcomes and would require a larger, prospective study, including determining whether newborns are truly smaller than their potential or are simply constitutionally small

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