Gestational diabetes (GDM) is a condition that manifests as high blood sugar levels (hyperglycemia) during pregnancy in previously healthy women. It develops as a result of increased maternal body's resistance to insulin - a major hormone that allows for utilisation of glucose (sugar taken in with food) within cells. It was found out that GDM occurs more frequently in overweight women but also in women with a history of certain conditions such as polycystic ovary syndrome (PCOS). Usually, GDM disappears after pregnancy is completed but it is associated with some serious hazards for women and her unborn child, if untreated properly. Diet is a first-choice treatment but sometimes insulin therapy must be initiated if keeping a diet alone is not enough to maintain blood sugar within recommended values. Insulin therapy is effective but it requires several injections during each day and insulin is a strong acting hypoglycemic agent that may induce rapid falls in blood sugar, also dangerous for mother and unborn child.

In the investigators study, the investigators would like to investigate if metformin that is a commonly used hypoglycemic drug can be effectively used for GDM treatment. Metformin has been used successfully for a long time to treat type 2 diabetes mellitus and PCOS and, according to current data, it is not dangerous neither for mother nor for baby when used during gestation.

Inclusion Criteria:

• diabetes diagnosed during pregnancy
• single pregnancy
• ineffective diet therapy

Exclusion Criteria:

• pregestational diabetes
• fetal malformation
• multiple pregnancy
• contraindications to metformin therapy (liver or kidney disease)