Diabetes mellitus complicates 1-2% of all pregnancies but is associated with high a perinatal morbidity and mortality. Gestational diabetes affects up to 4% of pregnancies and is associated with foetal macrosomia (large for dates). Foetal growth is a complex process influenced by genetics, maternal factors, uterine environment and maternal and foetal hormonal status. Infants of pre-gestational diabetic mothers have an additional influence of maternal fluctuations in glycaemia. The purpose of this paper is to review maternal and foetal growth factors, including insulin, in the aetiology of macrosomia in diabetic pregnancy. Placental Growth Hormone is the major growth hormone secreted during human pregnancy. Leptin may have a role in satiety. Resistin was originally proposed as the link between obesity and diabetes but is now thought to have a more complex role. These hormones and their actions on human foetal growth are reviewed in depth with particular reference to both pre-gestational (type 1 and type 2 diabetes) and gestational diabetes. Previously, increased foetal weight in infants of diabetic mothers was thought to be as a result of maternal hyperglycaemia. It is now evident that the control of foetal growth, in normal as well as diabetic pregnancies, is far more complex than previously understood.