a large margin of safety for fetal growth. Conversely, common clinical causes of impaired fetal growth such as maternal hypertension associated with reduced uterine blood flow, or placental infarcts resulting in reduced placental transfer capacity, may severely limit fetal nutrient supply without a corresponding change in maternal nutrition. Much confusion and debate in the literature about the relevance of nutrition to human fetal growth has arisen from failure to make this distinction between maternal nutrition (relatively easy to measure but relatively less important) and fetal nutrition (very difficult to measure but very important). Factors along the fetal 'supply line' which can mediate the differences between maternal nutrition and fetal nutrition. Much of our knowledge of fetal nutrition comes from studies in sheep, which are widely used in studies of fetal physiology because of their large size, long gestation and relative ease of surgical manipulation. These characteristics allow chronic fetal catheterization. Vascular catheters and other monitoring devices such as electrodes and growth measuring devices can be surgically placed in the ewe and fetus, the fetus returned to the uterus and the ewe allowed to recover. Fetal blood can then be sampled and the fetus studied over many days or weeks in the relatively undisturbed conditions of intrauterine life in vivo . Although there are important interspecies differences which must be considered, there are also remarkable similarities in the physiology of fetuses of different species. The fetal 'diet' of the late gestation mammalian fetus is remarkably consistent in different species, comprising glucose,