High pre-pregnancy BMI and excessive GWG are associated with many unfavourable maternal and neonatal outcomes and are independent risk factors for gestational diabetes mellitus (GDM) and large for gestatiola age (LGA) babies. Overweight/obese women should be counselled regarding their body weight before conception; however, most women have access to obstetricians only when they are pregnant. The Institute of Medicine (IOM) revised the guidelines of recommended GWG according to the BMI; however, only a minority of women succeed in reaching the recommended GWG. Among the interventions aimed at preventing excessive GWG, few have demonstrated efficacy in high-risk populations; the principal issues are population heterogeneity, the interventional methods, and the timing of the interventional programs. Additionally, lifestyle interventions did not have a substantial effect on other clinical outcomes. Dietary advice to prevent gestational diabetes mellitus (GDM) appears to be beneficial in general, although the results are overly heterogeneous. A systematic review concerning exercise alone demonstrated no effect on preventing GDM, whereas another study showed only a slight protective effect. The reports evaluating the efficacy of diverse approaches (exercise, diet, lifestyle interventions, dietary supplements) to prevent GDM are of poor quality. Adherence to lifestyle recommendations could be a major determinant of their efficacy, specifically among overweight/obese women. Previous studies investigated adherence to specific dietary patterns and their effect on pregnancy outcomes; however, no study has investigated adherence among overweight/obese pregnant women and its effect on the onset of GDM. Nowadays, there are insufficient evidences for recommend a specific diet in preventing LGA babies.