There is a growing body of information relating diverse diseases and the consequences of injury to generation and toxicity of reactive oxygen species (ROS). Recently, it has been shown that the fetus and its membranes are also vulnerable to this toxicity, suggesting that a number of obstetric diseases may result from exposure to ROS, which are ubiquitous in aerobic organisms. Endogenous antioxidants, including superoxide dismutase, catalase, and glutathione peroxidase are essential for defense against ROS. It is significant that all antioxidants appear to be down-regulated in the fetus and membranes, suggesting the possibility that any process that further depresses their activities or increases the burden of ROS may compromise fetal development or maternal health. When permitted to accumulate, ROS can damage all classes of macromolecules, including lipids, proteins, and nucleic acids. Toxicity includes mutation, protein degradation, and lipid peroxidation, which can severely disturb membrane permeability and alter intracellular calcium and pH. An understanding of the generation and toxicity of ROS should help to define their potential roles in obstetric disease and lead to innovative preventive and therapeutic approaches.