Background <p>Maternal malnutrition, expressed as both undernutrition and excessive gestational weight gain, remains a major public health challenge in Mexico. Primary health care units are the main platform for the provision of evidence-based nutritional interventions, including nutritional status assessment and counseling on healthy eating and physical activity. However, there is limited knowledge about their systematic implementation in routine prenatal care. This study aimed to evaluate the implementation of these interventions in primary health care units in Morelos, Mexico, focusing on key implementation outcomes.</p> Methods <p>A mixed-methods sequential design was applied. Quantitative data were collected through structured and validated questionnaires with health personnel (<i>n</i> = 59) to assess acceptability, adoption, and feasibility; review of physical records of prenatal consultations (<i>n</i> = 1,067) to estimate coverage; and non-participant observation of consultations (<i>n</i> = 368) to measure fidelity. Implementation outcomes were assessed using validated instruments and compliance checklists. Descriptive statistics and multiple linear regression analyses were performed. Qualitative data were obtained through in-depth interviews with health personnel and pregnant women (<i>n</i> = 24) from a subsample of 12 units between April and May 2025, analyzed thematically to explore perceptions, facilitators, and barriers.</p> Results <p>Health personnel expressed high acceptability and willingness to adopt nutritional interventions, although only 27% had received specific training in nutrition care. Coverage of weight measurement was nearly universal, but hemoglobin testing (24.6%) and nutritional status assessment (8.2%) had very low performance, particularly in rural units. Fidelity varied across interventions: nutritional assessment achieved the highest compliance (47.8%), while counseling on healthy eating (19.1%) and physical activity (8.6%) were rarely implemented. Structural barriers included limited consultation time, lack of educational materials, and excessive workload. Pregnant women valued counseling but reported economic and cultural challenges to adopting recommendations.</p> Conclusions <p>Although health personnel show strong acceptance of nutritional interventions, their implementation in primary care is hindered by systemic barriers. Low coverage and fidelity, especially in rural contexts, represent missed opportunities to improve maternal and child health. Greater training, resources, and integration of structured counseling into prenatal care are needed to strengthen equity and effectiveness of services.</p>