Abstract : It is becoming a public health concern to predict which pregnant women may develop gestational diabetes mellitus (GDM). The goal of this case control research is to investigate the role of maternal oxidative stress levels in the first, second, and third trimesters, as well as other factors, in the development of GDM. During October and December 2021, 142 women participated in this study. The 101 GDM patients were separated into three groups based on their trimester (T1, T2, and T3), and 41 healthy pregnant women were chosen as the control group. Levels of oxidative stress were determined by measurement of Total Antioxidant status (TAS) and Total oxidant status (TOS) using colorimetric methods while oxidative stress index (OSI) was calculated. Advanced oxidation protein products (AOPP), xanthine oxidase (XO) and dehydrogenase (XDH) activities were measured. The TOS levels were higher in all GDM groups compared with control (p=0.000) and high significant between patients groups in the order (T3>T2>T1), while high significant of TAS in GDM than control but in the order (T3<T2<T1). The mean levels of OSI were higher significantly in women with GDM in all trimester (T1=73.79 ± 18.43, T2=81.28 ± 18.06, T3=96.70 ± 20.69) in comparison with control (37.28 ± 18.95) with statistical significance (P = 0.000). Oxidative stress levels were higher significantly in the order T3>T2>T1, and the same results were found in the measured AOPP, Xanthine oxidase, XO\XDH ratio levels. On the other hand, Total protein, Albumin, Globulin, and Alb\Glo ratio levels showed non-significant difference (P>0.05) in all GDM groups when compared with that of healthy pregnant women. The correlation of oxidative stress with TOS in T1, T2, and T3 groups was positive with significance, while negatively correlated with TAS in T1, T2, T3 groups. From the findings of this study, a significant increase in total oxidant demonstrated by overall lower antioxidant level in GDM patients tend to heighten oxidative stress.