A prospective diagnostic accuracy study investigated metabolic biomarkers for the early detection of gestational diabetes mellitus (GDM) in 250 pregnant women in the first trimester (10–14 weeks) at Bandırma Onyedi Eylül University Hospital. Complete blood count, fasting glucose and lipid profile were recorded, from which NLR, PLR, MLR, NMR, LHR, MHR, GHR, AIP and TyG indices were calculated. GDM was diagnosed by 75 g OGTT in 24–28. weeks according to IADPSG criteria, with a prevalence of 18.0% (n = 45). Age, body weight, BMI, and parity were significantly higher in the GDM group (p < 0.05), but biomarker means were not significantly different between groups (p > 0.05). ROC analysis showed AUC values: NLR 0.470, PLR 0.490, MLR 0.474, NMR 0.481, LHR 0.567, MHR 0.506, GHR 0.466, AIP 0.76 and 0.56, none exceeded the threshold of 0.70. LHR, MHR, AIP and TyG indices have limited predictive value for GDM. OGTT remains the most reliable method, more specific biomarkers and multicenter studies are needed for early diagnosis.