Women with type 1 (T1DM) or type 2 diabetes (T2DM) diagnosed prior to pregnancy are classified as having pre-existing diabetes mellitus (DM). The prevalence of hyperglycemia in pregnancy has been estimated at 17% globally and 5.4% in Europe, differences existing among racial and ethnic groups, following the prevalence of type 2 diabetes. Only a minority (approximately 15%) of the cases of diabetes during pregnancy represent women with pre-existing diabetes. Because of the rising prevalence of obesity and limited screening for diabetes in young women, there has been, globally, an increase in the diagnosis of previously unknown overt diabetes early in pregnancy; these women should be treated as women with pre-existing diabetes, as they may already have unrecognized complications (e.g., nephropathy and retinopathy). The Hellenic Endocrine Society and the Hellenic Society of Maternal-Fetal Medicine commissioned an expert group to construct national guidelines on "Diabetes mellitus and pregnancy: Pre-existing type 1 and 2 diabetes mellitus". Following a search for the best available evidence and critical appraisal of the search results, the writing group generated a series of consensus recommendations regarding preconception counseling and care, care during pregnancy, and care after the pregnancy in cases of pre-existing T1DM and T2DM.