Oxidative stress (OS) plays a role in pregnancy at risk of pre-eclampsia, diabetes and premature labour. We measured three markers of OS: total antioxidant capacity (TAC), thiolyte capacity and pro-oxidant capacity in 45 women: 15 normal pregnancies, 17 pathological pregnancies (pre-eclampsia and pregestational diabetes) and 13 delivered pre-term. Plasma TAC (μmol/ml) values in patients with pathological pregnancies (235.67 ± 70.08) (p(1) = 0.0086) and pre-term labour (243.51 ± 50.52) (p(2) = 0.0479) were significantly reduced as compared with the controls (306.78 ± 70.08). Thiolyte capacity (μmol/ml) in the pathological pregnancies (326.03 ± 78.24) (p(3) = 0.0029) and in pre-term labour (335.94 ± 76.63) (p(4) = 0.0084) groups were significantly reduced compared with the control group (417.48 ± 39.76) (p < 0.05). Pro-oxidant capacity (mg/100 ml) in the pathological pregnancies (94.11 ± 26.13) (p(5) = 0.00034) and in pre-term labour (87.18 ± 20.28) (p(6) = 0.00044) groups were significantly higher compared with the controls (60.27 ± 6.33). Elevated OS values were seen in pathological pregnancies. This supports the important role of OS in diseases in pregnancy, particularly pre-eclampsia, diabetes and pre-term birth.