(338) Isolated small abdominal circumference and timing of delivery: prognostic implications for perinatal outcomes

Expanding the definition of fetal growth restriction to include abdominal circumference (AC) < 10th percentile aims to improve risk detection. We compared perinatal outcomes in pregnancies with isolated small AC (iAC), estimated fetal weight < 10th percentile (EFW10), and EFW ≥ 10th percentile (EFW10+); evaluated the influence of delivery timing; and assessed predictive performance.

Study Design:

We assembled a retrospective cohort of singleton, non-anomalous pregnancies with ≥ 1 third trimester (≥ 28 weeks) growth ultrasound performed between 2017 and 2024 at a single academic center. A composite outcome (stillbirth, fetal demise, small for gestational age, NICU stay, Apgar score at 5 minutes < 7, or C-section) was compared using logistic regression models with generalized estimating equations, adjusting for maternal demographics, comorbidities, and gestational age. The influence of delivery timing was evaluated with stratified analyses. Predictive performance was assessed via ROC curves and AUC.

Results:

iAC and EFW10 were identified in 2.9% (n=540) and 7.1% (n=1,322) of 18,771 pregnancies, respectively. The composite outcome occurred in 63.3% of iAC, 77.5% of EFW10, and 40.4% of EFW10+ groups. Compared to EFW10+, iAC was associated with increased odds of adverse outcomes [adjusted OR 3.07 (95% CI: 2.41–3.92)], though lower than EFW10 [OR 8.70 (6.93–10.91)]. Compared to delivery at 37 weeks, delivery at 38-39 weeks had higher odds of adverse outcomes in both iAC and EFW10 groups. Although ROC curve analysis showed statistically significant lower AUC for iAC (0.67 vs. 0.69, p < 0.001), both iAC and EFW10 had high sensitivity (98%) but low specificity (4% vs. 12%).

Conclusion:

These findings highlight the increased risk of adverse perinatal outcomes associated with isolated small AC, support increased fetal surveillance, and suggest that delivery timing may mitigate poor outcomes. Despite high sensitivity, the low specificity of iAC limits its standalone utility as a screening tool and underscores the need for careful interpretation alongside clinical context.