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Context: Metformin is widely prescribed to pregnant women with polycystic ovary syndrome (PCOS) in an attempt to reduce pregnancy complications. Metformin is not approved for this indication, and evidence for this practice is lacking. Objectives: Our objective was to test the hypothesis that metformin, from first trimester to delivery, reduces pregnancy complications in women with PCOS. Design and Setting: We conducted a randomized, placebo-controlled, double-blind, multicenter study at 11 secondary care centers. Participants: The participants were 257 women with PCOS, in the first trimester of pregnancy, aged 18–42 yr. Intervention: We randomly assigned 274 singleton pregnancies (in 257 women) to receive metformin or placebo, from first trimester to delivery. Main Outcome Measures: The prevalence of preeclampsia, gestational diabetes mellitus, preterm delivery, and a composite of these three outcomes is reported. Results: Preeclampsia prevalence was 7.4% in the metformin group and 3.7% in the placebo group (3.7%; 95% CI, −1.7–9.2) (P = 0.18). Preterm delivery prevalence was 3.7% in the metformin group and 8.2% in the placebo group (−4.4%; 95%, CI, −10.1–1.2) (P = 0.12). Gestational diabetes mellitus prevalence was 17.6% in the metformin group and 16.9% in the placebo group (0.8%; 95% CI, −8.6–10.2) (P = 0.87). The composite primary endpoint prevalence was 25.9 and 24.4%, respectively (1.5%; 95% CI, −8.9–11.3) (P = 0.78). Women in the metformin group gained less weight during pregnancy compared with those in the placebo group. There was no difference in fetal birth weight between the groups. Conclusions: Metformin treatment from first trimester to delivery did not reduce pregnancy complications in PCOS.