In England and Wales, about 2-5% of pregnancies are complicated with diabetes each year. Diabetes is a particular problem in the South Asian (SA) ethnic group with the prevalence of Type 2 Diabetes and GDM being about 6 times and 11 times higher respectively as compared to White British (WB) women. My PhD project was undertaken to study the influence of ethnicity and maternal hyperglycaemia during pregnancy on neonatal outcomes. This project consists of two retrospective studies and one prospective pilot study. The first retrospective study was undertaken to compare the neonatal outcomes in WB and SA infants born to mothers with gestational or pre-gestational diabetes (Type 1 and Type 2 diabetes). The second retrospective study was undertaken to compare the risk of morbidity and mortality between large for gestational age infants with a birthweight ≥ 97th centile and appropriate for gestational age infants with birthweight between 10th – 90th centile, both born to mothers without diabetes. Maternal hyperglycaemia during pregnancy leads fetal exposure to high blood glucose levels, which in turn leads to fetal hyperinsulinism. The neonatal complications seen in infants of diabetic mothers are due to persistent fetal hyperinsulinism after birth. Currently there is no clinical or biochemical test to identify, at birth, the infants who are at risk of neonatal complications. A prospective pilot study was undertaken to evaluate the feasibility of using cord blood C-peptide (surrogate marker of insulin) to identify infants born to mothers with diabetes and LGA infants of non-diabetic mothers at risk of postnatal complications. Such a test would enable early implementation of interventions to avoid complications and at the same time free the vast majority of infants from unnecessary medicalisation of their postnatal care.