offspring. Thus a very small animal with a short gestation, such as the guinea pig, has fetuses with a relatively high growth rate in late gestation (Table 2). Such fetuses must allocate a large proportion of total nutrient supply for growth. Limitations on nutrient supply will therefore have a much bigger effect on growth rate than they would in a larger species with a smaller relative growth rate. Similarly, in species with a large proportion of body fat at birth such as the guinea pig or human infant, a given rate of tissue acquisition will require a higher energy input because of the high energy density of the fat. Once again, this will increase the proportion of available energy supply which must be directed towards tissue growth. Such species would therefore be more vulnerable than other species to restrictions in energy supply in terms of their effects on fetal growth. 36 Factors influencing the effects of fetal undernutrition in different species There are also important species differences in placental structure and function. 51 -- 53 Species which are relatively fat at birth such as the guinea pig and human have placentas which are relatively permeable to fatty acids and related molecules in late gestation. Fatty acids may thus form a small but important component of the fetal 'diet' in these species at the end of pregnancy. 36 Maternal fasting in both sheep and women results in increased lipolysis and increased circulating ketone and fatty acid concentrations. In the sheep, ketones