© Oxford University Press 2004. All rights reserved. This chapter examines the sources of variation in fetal growth, in particular nutrition in the context of the fetal nutritional programming hypothesis. The influence of fetal and maternal genotype on fetal growth is considered and set in the context of non-genetic intergenerational influences on birthweight, such as the mother's height and her own birthweight. The concept of programming is considered both with reference to nutritional programming and the wider view of programming, with particular reference to the hormonal, metabolic, and circulatory milieu to which the fetus is exposed.