Objective: To evaluate serum concentration of 8-isoprostane, nitrotyrosine (NT), and total antioxidant capacity (TAC) in pregnant women with diabetes mellitus (DM) considering preconception planning and method of diabetes correction in 11–14 and 30–34 weeks. Materials and methods: The study included 130 women: T1DM (n = 40), T2DM (n = 35), gestational diabetes (GDM, n = 40) and the control group (n = 15). The serum concentrations of NT, 8-isoprostane, and TAC were measured by ELISA methods. Results: Elevated 8-isoprostane levels were observed in all patients with DM, but this biomarker's maximum values have been seen in T1DM and T2DM on insulin groups. A similar tendency was observed for the concentration of NT in both the 1st and 3rd trimesters. TAC levels showed a statistically relevant decrease in all DM groups compared to the control. The correlation analysis showed a direct correlation between HbA1c and serum 8-isoprostane levels in the 1st (r =.27) and 3rd (r =.3) pregnancy trimesters as well as inverse correlation with TAC level (r = −.48). Direct (NT, 8-isoprostane) and inverse correlations (TAC) were fixated for this biomarker concentration and preeclampsia rates. Conclusion: DM in pregnancy is related to oxidative stress activation, which might lead to the development of adverse perinatal outcomes.