Metformin is an oral insulin-sensitizing anti-diabetic drug. Polycystic ovary syndrome (PCOS) and gestational diabetes (GDM) are both associated with insulin resistance and hyperinsulinemia. Metformin can bring potential benefits in pregnant women due to its favorable metabolic effect. Nevertheless, there is a possibility of adverse effects on the fetus as metformin crosses the placenta. In this review we discuss safety and indications for metformin administration in pregnancy.