A prenatal diet affects materno-foetal outcomes. This is a post hoc analysis of the St. Carlos gestational diabetes mellitus (GDM) Prevention Study. It aims to evaluate the effect of a late first-trimester (>12 gestational weeks) degree of adherence to a MedDiet pattern-based on six food targets-on a composite of materno-foetal outcomes (CMFCs). The CMFCs were defined as having emergency C-section, perineal trauma, pregnancy-induced hypertension and preeclampsia, prematurity, large-for-gestational-age, and/or small-for-gestational-age. A total of 874 women were stratified into three groups according to late first-trimester compliance with six food targets: >12 servings/week of vegetables, >12 servings/week of fruits, <2 servings/week of juice, >3 servings/week of nuts, >6 days/week consumption of extra virgin olive oil (EVOO), and ≥40 mL/day of EVOO. High adherence was defined as complying with 5⁻6 targets; moderate adherence 2⁻4 targets; low adherence 0⁻1 targets. There was a linear association between high, moderate, and low adherence, and a lower risk of GDM, CMFCs, urinary tract infections (UTI), prematurity, and small-for-gestational-age (SGA) newborns (all < 0.05). The odds ratios (95% CI) for GDM and CMFCs in women with a high adherence were 0.35((0.18⁻0.67), = 0.002) and 0.23((0.11⁻0.48), < 0.001), respectively. Late first-trimester high adherence to the predefined six food targets is associated with a reduction in the risk of GDM, CMFCs, UTI, prematurity, and SGA new-borns.