The prevalence of Type 2 diabetes in women of childbearing age continues to grow as the incidence of Type 2 diabetes increases. Recent evidence shows that treatment of gestational diabetes ensures the best possible outcome for pregnancy complicated by gestational diabetes. Metformin is a logical treatment in these circumstances but there has always been concern about its safety for the fetus, particularly as it crosses the placenta and it may increase the risk of teratogenesis. Although evidence is accumulating that metformin is useful and has a role in polycystic ovary syndrome, a condition of insulin resistance, it is not yet accepted as treatment for Type 2 diabetes in pregnancy and gestational diabetes. Observational data supports the use of metformin in Type 2 diabetes in pregnancy and its role in gestational diabetes is currently under investigation. Metformin may become an important treatment for women with either gestational or Type 2 diabetes in pregnancy and indeed may have additional important benefits for women, including reducing insulin resistance, body weight and long-term risk of diabetes. There is a need for a randomized controlled trial in women with Type 2 diabetes in pregnancy with long-term follow-up of both mothers and children. Until then the best advice remains that optimized glycaemic control prior to conception and during pregnancy is the most important intervention for best possible pregnancy outcome.