The overall aim of the PregMet 2 Study is to investigate whether metformin prevents late miscarriages and preterm deliveries in PCOS women treated with metformin from first trimester of pregnancy to delivery in a large, randomized, controlled, multi-centre trial setting.

The investigators hypothesis is that metformin compared to placebo treatment from the first trimester to term, reduces the prevalence of late miscarriage (gestational week 13-22) and preterm birth (gestational week < 37) in PCOS women diagnosed according to Rotterdam 2003 consensus criteria, with singleton pregnancy.

Inclusion Criteria:

• Age: 18-45 years
• PCOS diagnosed or reconfirmed according to Rotterdam 2003 criteria, not more than 5 years prior to inclusion
• Mode of conception: Any
• Single, viable fetus, diagnosed by vaginal ultrasound
• Gestational week at inclusion: week 6 + 0 to 12 + 0
• Wash out for metformin: at least 7 days
• Able to communicate fluently in the official language at the study cite or English

Exclusion Criteria:

• Any type of diabetes (except GDM in former pregnancy)
• Known liver disease or ALAT > 100 IU/L
• Known kidney disease or creatinine > 110 μmol/L
• Known alcohol or drug abuse
• Use of drugs interfering with metformin: erytromycin (or other macrolides), cimetidine, anticoagulation therapy
• Unsuitable for participation of other reasons