One approach to prevent the rising burden of diabetes is to address the issue of gestational diabetes mellitus (GDM). GDM has a growing prevalence up to 5-10% (and even higher in specific subgroups), with a pregnant population becoming older and more obese worldwide. GDM increases the risks of complications during pregnancy, at delivery and on the longer term, like type 2 diabetes (T2D) and persistent obesity, in mother and child. Moreover, insulin added to diet as the standard care for GDM has disadvantages for mother (maintenance of hyperinsulinaemia, increasing weight and blood pressure) and child (macrosomia, hypoglycaemia) with related adverse pregnancy outcomes. Metformin, as an insulin sensitizer targeting the cause of GDM, may have essential benefits, as suggested by observational studies. However, RCTs with metformin (early positioned in the treatment of GDM) are still lacking. The POEM study is the first Randomized controlled trial (RCT) in GDM to test the hypothesis that metformin, early given from the start of the diagnosis GDM, on top of diet and lifestyle improves clinically relevant pregnancy outcomes in mother and child during pregnancy, at delivery and on the longer term - up to 20 years after birth.