First-Trimester Glycated Hemoglobin (HbA1c) and Maternal Characteristics in the Prediction of Gestational Diabetes Mellitus (GDM): A Prospective Cohort Study.

Objective: To assess the predictive performance of first-trimester HbA1c, both as an individual marker and in combination with clinical risk factors, for subsequent development of GDM. Methods: A prospective cohort study was conducted at a single tertiary center in Thailand. Singleton pregnant women between 10 and 14 weeks of gestation were consecutively recruited. Women with pregestational diabetes mellitus were excluded. Baseline clinical characteristics were systematically recorded, and blood samples were obtained for HbA1c measurement. GDM was diagnosed at 24-28 weeks of gestation using the standard two-step approach, 100-g OGTT with NDDG thresholds. The predictive performance of HbA1c alone and in combination with clinical risk factors was evaluated using ROC curve analysis. Results: Among 302 participants, 98 (32.5%) were diagnosed with GDM. Women who developed GDM were of more advanced maternal age and had higher pre-pregnancy BMI and body weight. They also more frequently reported a family history of diabetes mellitus and dyslipidemia. First-trimester HbA1c levels were significantly elevated in the GDM group. HbA1c alone demonstrated modest discriminatory performance (AUC=0.675). Incorporation of HbA1c into a multivariable model with maternal age and BMI improved predictive accuracy (AUC=0.726). At an probability cutoff 0.245, the combined model achieved a sensitivity of 80.6% and a specificity of 51.0%. Conclusion: First-trimester HbA1c levels are significantly elevated among women who subsequently develop GDM. Although HbA1c alone provides modest discriminatory capacity, integrating HbA1c with maternal age and pre-pregnancy BMI substantially enhances predictive performance. These findings support the application of this straightforward combined model for early pregnancy risk stratification.