Intrauterine growth retardation (IUGR) is characterized by the failure of the fetus to grow at a normal rate in utero and is associated with a number of endocrine and metabolic changes. Our knowledge of the placental nutrient supply and the endocrine status of the fetal-placental unit during pregnancies involving IUGR has greatly increased over the past decade as a result of the availability of fetal blood samples obtained under relatively steady state conditions. These studies have provided evidence that the supply of glucose is impaired only under severe conditions, whereas placental transfer of amino acids is reduced even in fetuses with normal oxygenation and feto-placental blood flow. Moreover, significant in utero relationships have been reported between fetal weight and circulating levels of growth factors such as insulin-like growth factor-I and leptin. When measured per kg fetal weight, however, levels of leptin are significantly higher in growth-retarded fetuses, with abnormal feto-placental blood flow and reduced oxygen content. The metabolic and endocrine changes observed should be interpreted in relation to the severity of the disease.