Myo-Inositol is classified as a member of the vitamin B complex. It is a constituent of living cells and is widespread in many food. It is involved in a number of biological processes, including insulin signal transduction, resulting in modulating insulin sensitivity. This is a prospective, randomized, double center, placebo-controlled study. Two hundred and twenty obese pregnant women will be included in the trial and, after an informed consent, will assume randomly 2 g of myo-inositol twice a day or placebo from 12-13th weeks gestation until delivery. Then,they will perform an Oral Glucose Tolerance Test (OGTT) at 24-28th weeks gestation. Records about delivery (gestational age, neonatal weight...) will be registered.