The aim of this study was to investigate the oxidative stress status during the second and third trimester of pregnancy in patients with GDM by determining plasma levels of 8-iso-prostaglandin F2α (8-iso-PGF2α) as a marker of lipid peroxidation, advanced oxidative protein products (AOPPs) and protein carbonyl (PCO) as markers of protein oxidation, and plasma glutathione peroxidase-3 (GPX-3) and paraoxonase (PON1) as markers of antioxidative defense, to explore the role of oxidative stress in the development and progression of GDM. The gene discussed is PON1; the disease is gestational diabetes.