Fetal nutrients are derived largely from the mother, and fetal nutrition is thus closely related to maternal nutrition. However, it is important to appreciate that maternal nutrition is not the same as fetal nutrition. Firstly, the mother has her own nutrient demands which may be in conflict with those of the fetus. For example, pregnant adolescent sheep deliver smaller fetuses, especially when the ewes are very well nourished and therefore growing well, and the growth restriction appears to be predominantly secondary to reduced placental growth. Human adolescents also tend to give birth to lighter infants, and birth weight has been reported to be less in offspring of adolescents with a higher dietary sugar intake. Secondly, the fetus lies at the end of a long supply line which can be impaired at many points. Nutrients are used by the fetus predominantly for growth and metabolism, with little energy expenditure on other processes such as thermoregulation, movement and digestion. Fetal nutrients are in fact the main drivers of fetal growth, with genetic factors playing a much smaller role. Indeed, the genetic regulation of fetal growth itself appears to be under nutritional regulation, with levels of all the major hormones involved in fetal growth being regulated by circulating nutrient levels. The placenta is also a very metabolically active organ with its own nutrient demands and metabolic pathways.