Predictive value of routine ultrasound indicators in the second trimester for gestational diabetes mellitus: a retrospective cohort study

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Background: Gestational diabetes mellitus (GDM) is associated with adverse maternal and offspring outcomes, yet the predictive value of routine second-trimester ultrasound parameters remains unclear. This study aimed to systematically evaluate the ability of common fetal ultrasound indices measured at 19-24 weeks to predict GDM. Methods: This retrospective cohort study initially enrolled 254 singleton pregnancies. After exclusions, 241 women (117 with GDM, 124 controls) were analyzed. Nine ultrasound parameters (biparietal diameter, head circumference, abdominal circumference, femur length, amniotic fluid depth, fetal heart rate, humeral length, placental thickness, and estimated fetal weight) were converted to gestational age-adjusted Z-scores. Individual and combined predictive performances were assessed using ROC curves, multivariable logistic regression, calibration curves, and decision curve analysis (DCA). Results: Among single ultrasound indices, only amniotic fluid depth showed statistical significance, albeit with a modest AUC of 0.588 (95% CI: 0.517-0.658, P = 0.017). After adjusting for pre-pregnancy BMI, amniotic fluid depth remained independently associated with GDM but in an inverse direction (OR = 0.754, 95% CI: 0.575-0.989, P = 0.041). The combined model (pre-pregnancy BMI plus six ultrasound Z-scores) achieved an AUC of 0.692 (95% CI: 0.625-0.758, P< 0.001), with good calibration (Hosmer-Lemeshow P = 0.339). DCA showed a modest positive net benefit within the threshold probability range of 0.1-0.3. Conclusion: Routine fetal ultrasound parameters at 19-24 weeks have very limited predictive value for GDM. The combined model including pre-pregnancy BMI failed to reach the clinically useful AUC of 0.70. These routine indices are not recommended for GDM screening before 24 weeks. Current practice of OGTT at 24-28 weeks should remain the standard. Future research should focus on later gestational windows or more specific sonographic biomarkers such as fetal pancreatic circumference and liver volume.

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