Oxygen is essential to the life of aerobic organisms: it enabled cell respiration and ATP synthesis, but can also become harmful for cells, as proposed in the oxygen paradal Approximately 90% of oxygen is metabolized to water the mitochondria while the remaining 10% are partial oxidized and produce reactive oxygen species (ROS) a reactive nitrogen species (RNS). The cell produces physiological quantities of ROS that play a major role in cell signaling a in the immune response to pathogens. When this production becomes excessive and overcomes the organism's antioxidation capacities, there is imbalance leading to oxidative sire Oxydative stress may result both from an increased production of ROS and from a defect in ROS detoxification pathway in particular anti-oxidant enzymes. Gene expression antioxidant enzymes, however, can also be stimulated oxidative stress. In the context of mammalian pregnancy, high levels of oxidative stress will induce cellular death and abortion whereas moderate amounts of oxidative stress can induced cellular dysfunction and lead to teratogenesis. In human oxidative stress has been pointed as playing an important in early spontaneous abortion, intra-uterine growth restriction pre-eclampsia, Down syndrome and premature delivery.