During pregnancy, several metabolic changes are observed which aim to provide optimum substrate, energy and other nutritional requirements to both the mother and the fetus. Maternal nutrition is the only source for most nutrients, influencing neonatal and placenta development, mother's physiological adjustment and also playing a major role in the destiny of the offspring. Over-nutrition or malnutrition are both linked with increased risk of diabetes mellitus in the offspring. Diabetes in pregnancy is the most common and important metabolic dysfunction in pregnancy. This is divided into two types and it is very important to distinguish between them, as each has different nutritional requirements and a different impact on the course of the pregnancy and the development of the fetus. Gestational diabetes mellitus (GDM) is the main type of diabetes in pregnancy, it usually appears in the second half of pregnancy and mainly influences fetal growth rate and can slow systemic development. Most women with GDM are treated with nutritional management alone. Pre-existing diabetes mellitus is present before pregnancy and it's effects begin at fertilization and implantation, and continue throughout pregnancy and thereafter. It involves high risk of early abortion, severe congenital defects and disrupted organogenesis. Although the treatment of pre-existing diabetes is usually a pharmacological one (insulin or oral pharmacological agents), nutritional management is still very important in normalization of glucose levels before and throughout the pregnancy. Fetal morbidity is lower in women with diabetes in pregnancy when optimal glucose control is maintained. Normalization of glucose levels during pregnancy is agreed to be the main factor in preventing poor outcomes in pregnancy. Dietary advice throughout pregnancy include frequent small meals which contain carbohydrates that are not highly processed, rich with slowly absorbed starches and non-soluble polysaccharides and with a low glycaemic index. The recommended daily caloric intake is individually suited to every woman. The main goals of nutritional management are to maintain balanced glucose levels and to provide enough energy and nutrients for all pregnant women, while avoiding ketosis, and minimizing the risk of hypoglycemia (in women treated with insulin). Health care providers should use the window of opportunity of pregnancy to change dietary patterns and to replace them with a healthy lifestyle for both the mother and her family.