Association Between Maternal Serum 25-Hydroxyvitamin D Concentrations and Gestational Diabetes Mellitus in Women Undergoing Diagnostic 100-g Oral Glucose Tolerance Testing

Background: Gestational diabetes mellitus (GDM) is a common metabolic complication of pregnancy. Vitamin D has been implicated in glucose metabolism; however, its association with GDM remains controversial. Objective: The objective was to evaluate the association between maternal serum 25-hydroxyvitamin D [25(OH)D] concentrations and GDM in pregnant women referred for diagnostic 100 g oral glucose tolerance testing following a positive 50 g glucose challenge test. Methods: This retrospective study included 92 pregnant women (44 with GDM and 48 without GDM) who underwent diagnostic 100 g oral glucose tolerance testing between January 2016 and June 2025 at a tertiary-care hospital. Demographic characteristics and biochemical parameters, including serum 25(OH)D concentrations, were obtained from medical records. Logistic regression analysis was performed to identify factors independently associated with GDM. Results: Women with GDM were significantly older than those without GDM (37.55 ± 6.06 vs. 29.21 ± 4.29 years, p < 0.001) and had lower serum vitamin D concentrations (14.08 ± 9.88 vs. 19.06 ± 10.30 ng/mL, p = 0.003). In multivariable logistic regression analysis, advanced maternal age (OR 1.404, 95% CI 1.217–1.620; p < 0.001) and lower serum vitamin D concentrations (OR 0.933, 95% CI 0.880–0.989; p = 0.020) were independently associated with GDM. Conclusion: Lower maternal serum 25(OH)D concentrations were independently associated with GDM in women referred for diagnostic 100 g oral glucose tolerance testing. These findings should be interpreted as observational and hypothesis-generating. Larger prospective studies are needed to confirm these results.