OBJECTIVE: Gestational diabetes mellitus (GDM) is the most prevalent medical complication in pregnancy. Early diagnosis of GDM can influence maternal/neonatal outcomes. To assess the association between platelet and blood inflammatory indices and the risk of GDM occurrence using the complete blood count (CBC) test. We also aimed to determine the sensitivity of each parameter for an early screening of this disorder during pregnancy.METHODS: This case-control study included 2 groups of 110 pregnant women with and without GDM. The women in each group were compared after the routine screening for GDM and after the CBC test at 24–28 weeks' gestation after being matched according to the inclusion criteria. Data were analyzed using SPSS version 16 and Medcalc version 14.8.1 software.RESULTS: There were statistically significant intergroup differences in white blood cell (WBC) count, platelet count, mean platelet volume (MPV), plateletcrit (PCT), MPV to platelet ratio, platelet to lymphocyte ratio, and Rh values. The values of lymphocyte count, neutrophil count, neutrophil to lymphocyte ratio, and blood group were not significantly different between groups. The logistic regression analysis showed the predictive values of WBC, platelet, MPV, and PCT in GDM. According to the receiver operating characteristic curve for all 3 variables, the level below the PCT chart was more than that of the others.CONCLUSION: Increasing platelet and inflammatory indices on the CBC test in the second trimester of pregnancy seemed to be associated with the probability of GDM occurrence.