Metformin, a drug developed for the treatment of patients with type II diabetes, has become commonly prescribed medication for PCOS patients. Initially, metformin was prescribed for patients with impaired glucose tolerance at the pre conception period, however more recently its use was expanded to many of the PCOS patients and for the whole duration of pregnancy. Several studies examining the effects of Metformin during pregnancy reported a lower pregnancy loss, reduced gestational diabetes and no increased risk for birth defects, however, several more recent studies also raised concerns about its safe use. The therapeutic effect of metformin stems from its ability to inhibit the action of the first complex of the electron transport resulting in reduced ATP production. At the initial stages of embryo development, the only source of ATP is the mitochondrial electron transport chain. Lowering ATP production at the critical stage of early embryo development may impair oocyte maturation and embryo development as well as reprogram the metabolic characteristics of the offspring.