Polycystic ovary syndrome (PCOS) is associated with insulin resistance, infertility, obesity and gestational complications. Metformin is widely used in fertility treatment of women with PCOS, due to a suggested positive effect of continued metformin treatment beyond the first trimester on pregnancy complications. Larger randomized trials have failed to confirm this. Metformin treatment has not been found to be superior to insulin treatment in women with gestational diabetes and may be associated with long-term consequences in the children in the form of overweight and disturbed glucose metabolism.