An early antenatal dietary intervention could play an important role in the prevention of metabolic diseases postpartum. The aim of this study is to evaluate whether an early, specific dietary intervention reduces women’s cardiovascular risk in the “fourth trimester”. This prospective cohort study compares 1675 women from the standard-care group (ScG/n = 676), who received standard-care dietary guidelines, with the intervention group (IG/n = 999), who received Mediterranean diet (MedDiet)-based dietary guidelines, supplemented with extra-virgin olive oil and nuts. Cardiovascular risk was determined by the presence of metabolic syndrome (MetS) and insulin resistance syndrome (IrS) (HOMA-IR 3.5) at 12–14 weeks postpartum. MetS was less frequent in the IG (11.3 vs. 19.3%, p < 0.05). The intervention was associated with a reduction in the relative risk of having MetS: 0.74 (95% CI, 0.60–0.90), but not in the risk of IrS. When analyzing the presence of having one or more components of the MetS, the IG had significantly higher rates of having 0 components and lower rates of having ≥1 (p-trend = 0.029). An early MedDiet-based nutritional intervention in pregnancy is associated with reductions in postpartum rates of MetS.