The lack of knowledge about the mechanisms of IUGR has prevented the development of effective therapeutic options, such that the current management of growth-restricted infants is empirical and is primarily aimed at selecting a safe time for delivery (2). Because nutritional and developmental research often involves invasive tissue collections and surgical procedures, it is neither ethical nor practical to conduct these experiments with the human placenta and fetus. Thus, animal models (e.g., mice, rats, pigs, and sheep) are instrumental for defining the mechanisms of IUGR and developing therapeutic means. Available evidence, which is discussed in the following sections, suggests that arginine [a nutritionally essential amino acid for the fetus (19)] plays a key role in development of the conceptus (embryo/fetus, associated placental membranes, and fetal fluids).