OBJECTIVE: The aim of this study was to investigate the role of systemic immune-inflammation index, neutrophil–lymphocyte ratio, lymphocyte–monocyte ratio, and platelet–lymphocyte ratios calculated in the first trimester as inflammatory markers in predicting gestational diabetes mellitus diagnosis. METHODS: This study was conducted retrospectively at a tertiary center between January 2020 and June 2023. A total of 111 pregnant women with gestational diabetes and 378 pregnant women in the control group were included in the study. Systemic immune-inflammation index, neutrophil–lymphocyte ratio, lymphocyte–monocyte ratio, and platelet–lymphocyte ratios values were compared between the gestational diabetes mellitus group patients and the healthy group. Receiver operating characteristic analysis curve was used for predicting gestational diabetes mellitus using systemic immune-inflammation index and lymphocyte–monocyte ratio. RESULTS: In pregnant women in the first trimester, systemic immune-inflammation index and lymphocyte–monocyte ratio values based on routine complete blood count parameters were found to be statistically significantly higher in gestational diabetes mellitus patients compared to healthy patients, while neutrophil–lymphocyte ratio and platelet–lymphocyte ratios values were found to be similar (p=0.033, p=0.005, p=0.211, and p=0.989). For predicting gestational diabetes mellitus, a cut-off value of 655.75 for systemic immune-inflammation index resulted in 80.2% sensitivity and 34.4% specificity, and a cut-off value of 3.62 for lymphocyte–monocyte ratio resulted in 56.8% sensitivity and 63.2% specificity, indicating good discriminatory ability. CONCLUSION: We believe that systemic immune-inflammation index and lymphocyte–monocyte ratio values measured in the first-trimester complete blood count parameters are effective in predicting gestational diabetes mellitus but are not effective in determining insulin requirement.