This chapter presents the study of intrauterine mortality in humans that is fraught with unavoidable difficulties of measurement and analysis. The terms intrauterine mortality and fetal loss are used synonymously and solely with reference to naturally occurring death in utero, that is, to spontaneous rather than induced abortion. Two major methodological problems plague the study of fetal loss: underreporting and selectivity bias. The first arises primarily because of the near impossibility of detecting early pregnancies, the second because of difficulties in choosing a sample of women representative of some larger population with respect to a number of risk factors predisposing to fetal loss. Some of the environmental factors that have been identified in various studies as increasing the risk of late fetal loss are infections (e.g., amniotis), radiation, cigarette smoking, alcohol and caffeine consumption, exposure to toxic chemicals, and maternal nutritional status, including both undernutrition and obesity.