Diabetes mellitus is one of the most common and important medical complications affecting pregnancy. It can predate the pregnancy ('pre-existing diabetes') or arise during pregnancy ('gestational diabetes', GDM). Typically, GDM resolves once the pregnancy has ended. However, about 3% of women with a diagnosis of GDM have type 2 diabetes diagnosed for the first time in pregnancy, which persists beyond pregnancy. The coexistence of diabetes of any type and pregnancy is associated with an increased risk of adverse outcomes for both the woman and baby. However, with appropriate management by a multidisciplinary team before, during and after delivery these risks can be minimised. Optimising blood glucose control, screening for maternal and fetal complications and a discussion about delivery are key strategies. During pregnancy, all women should be offered screening for GDM. After pregnancy, all women with GDM should be offered annual screening to identify the development of type 2 diabetes.