Objective: This study is a Narrative Review that aims at investigating the implications of obesity, excessive gestational weight gain (GWG) and gestational diabetes mellitus (GDM). Additionally, this Review seeks to explore the effectiveness of nutrition, and/or exercise interventions during pregnancy on reducing GWG and preventing GDM. Materials and Methods: The search in literature included studies that identified obesity, GWG, GDM and associated risks during pregnancy. Also, SR and MA focusing on interventions including diet, or physical activity (PA), or combined (i.e., lifestyle interventions) and their impact on metabolic risk during pregnancy, were identified through searches in PubMed, Cochrane Database of Systematic Reviews (CDSRs), and Scopus. Results: The study findings suggest that lifestyle interventions during pregnancy may be effective in reducing excessive GWG. Regarding the prevention of GDM, results from studies evaluating lifestyle interventions vary. However, significant and less controversial results were reported from studies assessing the efficacy of exercise interventions, particularly in high-risk pregnant women. Conclusions: Lifestyle interventions during pregnancy may reduce excessive GWG. Exercise during pregnancy may prevent GDM, especially in high-risk pregnant women. Future research is warranted to tailor lifestyle interventions for optimal effectiveness during pregnancy.