PCOS (cases) and non-PCOS (controls) pregnant women were enrolled. Cases will be randomized in four treatment groups: metformin plus placebo (group A1), metformin plus folic acid (group A2), placebo plus folic acid (group A3), placebo alone (group A4). Controls will not received any treatment (group B). Clinical and biochemical assessment (including serum markers of implantation), Doppler velocimetry of the uterine arteries, and trophoblastic invasion at histological and immunohistochemical evaluation.