Accuracy of third trimester ultrasound in predicting neonatal birthweight in patients with inflammatory bowel disease

Objective: To evaluate the accuracy of third trimester ultrasound in predicting birthweight in patients with inflammatory bowel disease (IBD) using the gestation-adjusted projection (GAP) method. Study design: Retrospective cohort study including pregnant patients with IBD who had third trimester ultrasounds and delivered at a single institution from 2012 to 2017. Controls included pregnant patients without IBD seen during the study period with third trimester ultrasounds. Correlation plots of GAP birthweight and actual birthweight (AB) were created for IBD-positive cases, IBD-negative controls, and IBD-positive cases with and without prior abdominal surgery. GAP predicted birthweight error was calculated for cases and controls. Univariable linear regression models estimated the association between predicted birthweight and AB. Multivariable linear regression models estimated the association between GAP birthweight and AB adjusting for age, BMI, race, and IBD status. Results: 320 patients were included (172 cases and 148 controls). Cases were more likely to be older (p < 0.001), white (p < 0.001), and have a lower BMI (p = 0.001). Correlation plots of GAP birthweight and AB showed linear correlations in cases (Spearman ρ = 0.81), controls (ρ = 0.74), cases with (p = 0.78) and without prior surgery (ρ = 0.83). GAP birthweight was significantly associated with AB in controls and cases in univariable linear regression models (β = 0.85, standard error = 0.04, p < 0.001; β = 0.90, standard error = 0.06, p < 0.001, respectively). No significant difference was found between the parameter estimates of the two models (p = 0.47). GAP birthweight remained significantly associated with AB in a multivariable linear regression model (β = 0.86, standard error = 0.03, p < 0.001). There were no significant differences between GAP predicted birthweight error between controls and cases (APE 11% vs 10% respectively, p = 0.56) and between cases without and with prior surgery (APE 10% vs 11%, p = 0.7). Conclusion: The accuracy of fetal biometry in the third trimester for predicting actual birthweight was equivalent between patients with and without IBD and those with prior abdominal surgery.