Maternal adaptations in energy metabolism are required to respond to the additional energy demands of pregnancy, and these adaptations are influenced by the degree of maternal obesity. Glucose intolerance does not influence the adaptation of maternal energy metabolism to pregnancy. Pregnancy is associated with increased maternal insulin resistance, which is further increased in obese and glucose-intolerant mothers. Endothelial function is improved in pregnancy but to a lesser extent in obese and glucose-intolerant mothers. While these changes in maternal metabolism and physiology in response to pregnancy have been described, the timing and the resulting impact on the quality of circulating maternal metabolites are poorly understood. An appreciation of the detail of these processes is important in order to inform our understanding of how obesity and glucose intolerance may contribute to adverse pregnancy outcome and to allow the development of sensible, evidence-based prevention strategies.