The changes in maternal physiology support the diverse needs of the developing fetus and prepare the gravida for the parturition. The aim of our study was to determine the serum levels of the products of reactive oxygen species, antioxidants and nitric oxide in healthy pregnant women at different gestational ages. The study has the approval of ethics committee of University of Medicine and Pharmacy, Cluj-Napoca and the approval of patients. We included in our study 30 healthy pregnant women (aged between 17 and 28 years) with single pregnancy that presented at the Department of Obstetrics Cluj-Napoca, for prenatal visits. We took blood samples from these patients at 6-8 weeks of gestation, 12-15 weeks, 20 weeks and at term before the onset of labor. As controls, we selected for our study 30 healthy non-pregnant women aged between 19 and 34 years. We established the serum level of the products of reactive oxygen species for all patients: lipid peroxides-Satoh method (1) and the carbonyl content of proteins-Reznik method (3). We also established the hydrogen donating ability of serum (that reflects the serum antioxidant capacity)-Hatano method (1) and the serum level of nitrate and nitrite (products of nitric oxide degradation in organism)-Griess reaction (4). All results are expressed as the mean ± S.D. The comparison between each group of pregnant women and the group of non-pregnant women was carried out using Student’s t test. A p value <0.05 was considered significant. We found that serum products of reactive oxygen species in healthy pregnant women of 6-8 (lipid peroxides 1.8±0.05 nmol/ml and serum carbonyl content of proteins 1.22±0.07 nmol/mg protein) and of 12-15 weeks (lipid peroxides 1.82±0.08 nmol/ml and serum carbonyl content of proteins 1.28±0.08 nmol/mg protein) do not differ significantly from those of healthy non-pregnant women (lipid peroxides 1.8±0.07 nmol/ml and serum carbonyl content of proteins 1.25±0.07 nmol/mg protein). In contrast, in women at 20 weeks we found significantly higher values (p<0.01) of these parameters (lipid peroxides 2.15±0.11 nmol/ml and serum carbonyl content of proteins 1.78±0.07 nmol/mg protein) and this increase was maintained until the end of the pregnancy. In addition, the hydrogen donating ability of serum was significantly increased (p<0.01) at 20 weeks (50.1±2.4%) compared with non-pregnant women (39.7±1.9%). The levels of nitrates and nitrites increased significantly (p<0.01) beginning at 12-15 weeks (19.9±1.8 μmol/ml) compared with the levels in non-pregnant women (12.2±1.04 μmol/ml) and was maintained until the end of the pregnancy (20.1±1.6 μmol/ml). We conclude that there is a new oxidative status in healthy pregnant women. The mother produces reactive oxygen species in greater quantities from 20 weeks until the end of the pregnancy but also has stronger antioxidant mechanisms. On the other hand, nitric oxide, known to produce relaxation of vascular and non-vascular smooth muscle (2), is increased earlier, namely from the beginning of the second trimester.