The increasing prevalence of obesity during women's reproductive years highlights the need for effective strategies to improve nutritional literacy as a preventive approach to adverse postnatal outcomes. This study outlines a protocol for a pilot randomized controlled trial assessing the potential of nutritional education sessions promoting the Mediterranean Diet (MedDiet) to improve adherence to this diet and evaluate its effects on cardiometabolic outcomes, namely on cardiac reverse remodeling, glycemic and blood pressure control, as well as on preconception weight recovery and changes in the fecal microbiota composition in pregnant women with obesity, hypertension, and/or diabetes. Secondarily, this study aims to evaluate how this dietary intervention indirectly interferes with infants' exclusive breastfeeding and the introduction of complementary feeding. It includes a face-to-face session using an eye-catching 13-page manual, followed by five monthly newsletters sent by e-mail. Participants will be monitored at one, six, and twelve months postpartum to evaluate the intervention's effectiveness. Findings from this study will contribute to the development of an evidence-based nutritional education tool designed to improve maternal health outcomes, with potential applicability for both health professionals and pregnant and postpartum women.