Objective To evaluate the effect of pregestational metformin administration on abortion risk in polycystic ovary syndrome (PCOS) patients. Design Systematic review and meta-analysis. Setting Academic Department of Obstetrics and Gynecology in Italy. Patient(s) Women affected by PCOS receiving pregestational metformin. Intervention(s) Systematic review of randomized controlled trials (RCTs) published up to June 2008 and subsequent meta-analysis. Main Outcome Measure(s) Abortion rate. Result(s) Seventeen RCTs were included in the final analysis. Overall, no effect of the metformin administration was detected on the abortion risk. No statistically significant effect of metformin on the abortion risk was observed in any comparison when subgrouping the RCTs according to received treatment. Conclusion(s) Metformin has no effect on the abortion risk in PCOS patients when administered before pregnancy.