During the past ten years the work of the Samuel S. Fels Research Institute has included a study of the growth, physiology and nutrition of the human fetus in utero. During this period my associates and I, in addition to collecting and publishing a considerable amount of research data, have made certain observations on fetal nutrition which I believe to be both interesting and significant. These observations have led to the formulation of the following hypothesis: The degree of intrauterine fetal activity or movement may be an important factor in the storage of fat by the fetus and therefore in the weight and state of nutrition of the newborn infant, and there are extrauterine factors, that might be called factors of prenatal environment, which are extremely important in stimulating the fetus to muscular movement and in consequently influencing the state of fetal metabolism and nutrition. Although the process of accumulating