To investigate the association between maternal oxidative stress at mid-gestation and subsequent development of pregnancy complications. A total of 503 healthy pregnant women provided their blood and urine samples at 24 to 26 weeks of gestation and were prospectively followed through postpartum. These samples were used to assess a variety of oxidative stress markers, including plasma total antioxidant capacity, 8-isoprostane, erythrocyte glutathione peroxidase and superoxide dismutase activity, and urinary 8-hydroxydeoxyguanosine (8-OHdG). Compared with women with uncomplicated pregnancies, significantly higher plasma 8-isoprostane levels were noted in women who developed preeclampsia (P = .008) and small-for-gestational age infants (P = .002), while higher urinary 8-OHdG concentrations were noted in women who subsequently had low-birth-weight neonates (<2500 g, P = .043). Increased maternal oxidative stress at mid-gestation was associated with subsequent pregnancy complications.