Pregnancy is associated with increased oxidative stress, and exaggeration of oxidative damage is considered important in pregnancy complications such as preeclampsia. There is, however, only scattered information regarding the longitudinal change of oxidative stress during pregnancy. We surmised that the placenta is the cause of the increased oxidative stress during pregnancy and hypothesized that the amount of placenta-derived cell-free fetal DNA in maternal plasma is correlated with maternal oxidative stress status. Blood and urine samples were collected at different stages of gestation and 6 to 8 weeks postpartum from 105 women with uncomplicated pregnancies. Additionally, 40 healthy women of reproductive age, but not pregnant, were recruited as controls. All oxidative stress markers, including urinary 8-hydroxydeoxyguanosine (8-OHdG), plasma 8-isoprostane, total antioxidant capacity (TAC), and erythrocyte glutathione peroxidase (GPX) and superoxide dismutase (SOD) activities, were increased in the third trimester, and most of them returned to nonpregnant levels postpartum. Furthermore, the DYS14 gene of the Y chromosome was quantified from plasma samples obtained at 26 to 30 weeks' gestation in 54 women who later delivered a male infant. Significant associations were noted between plasma concentrations of the DYS14 gene and the levels of urinary 8-OHdG and plasma 8-isoprostane, suggesting an association between the breakdown of syncytiotrophoblast and maternal oxidative stress during pregnancy.