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Objectives To check the hypothesis that myo-inositol supplementation, an insulin sensitizing substance, may reduce gestational diabetes mellitus (GDM) onset and insulin resistance in obese pregnant women. Methods A prospective, randomized, double center, open-label study was carried out in obese (Body Mass Index ⩾ 30 Kg/m 2 ) pregnant outpatients who were treated from the end of the first trimester with 2 g myo-inositol plus 200 μg folic acid twice a day ( n  = 110) or with only 200 μg folic acid twice a day (placebo group, n  = 110). The main outcome measures were the incidence of GDM and the variation of insulin resistance status calculated as the difference of Homeostasis Model Assessment of Insulin Resistance (Δ HOMA-IR), between the first trimester and the diagnostic Oral Glucose Tolerance Test (OGTT). Secondary outcomes were: the prevalence of fetal macrosomia (fetal weight >4000 g at delivery), caesarean section, gestational hypertension, preterm delivery, shoulder dystocia, and neonatal transfer to the Neonatal Intensive Care Unit (NICU). Results Incidence of GDM was significantly reduced in the myo-inositol group compared to the placebo group (15.5% vs. 34.6%, respectively, p  = 0.002). Consequently, in the myo-inositol group, a reduction of GDM risk occurrence was highlighted (OR = 0.34, 95% CI 0.17–0.68). Furthermore, women treated with myo-inositol showed a higher reduction of the Δ HOMA-IR compared to the control group (−0.79 ± 1.6 vs. 0.25 ± 1.4 respectively, p p  = 0.02) and cases of babies transferred to the NICU (0% vs. 4.8%, p  = 0.02). Conclusions Myo-inositol supplementation in obese pregnant women may reduce GDM incidence through a reduction of insulin resistance. Pregnancy outcome was improved, reducing gestational hypertension occurrence and cases of babies transferred to the NICU Disclosures R. D’Anna: None. A. Santamaria: None. F. Corrado: None. A. Di Benedetto: None. E. Petrella: None. F. Facchinetti: None.