Childhood obesity is a public health concern. Evidence-based, multicomponent, parenting interventions targeting early childhood and adapted to families' socioeconomic context are needed to prevent childhood obesity.

This study aims to evaluate the effectiveness of an early intervention in primary care for the prevention of childhood obesity, targeting vulnerable families where the mother and/or the father has overweight or obesity.

More specifically, the primary aim of this study is to evaluate the effectiveness the intervention to maintain a healthy weight in children aged 6 months to 3 years, measured by the BMI-for-age z-score at baseline, 6, 12, and 18 months, compared to usual care.

The study also aims to evaluate the effectiveness of the intervention, compared to usual care, for: promoting responsive feeding, improving dietary intake, improving parents' and children's physical activity levels, improving parents' eating styles, and improving children's sleep.

Finally, the study aims to determine the feasibility of implementing the intervention, and to identify factors influencing change and contextual factors in the implementation process.

A pragmatic cluster randomized controlled trial will be conducted. Approximately 526 families (parent-child family dyads) will be recruited, belonging to 76 primary care centers. Primary care centers will be randomly assigned to either an intervention or control group.

The intervention has been previously co-designed with families and healthcare professionals, and consists of five weekly sessions and 4 monthly sessions, lasting 2 hours each. It will be delivered by previously trained healthcare professionals or by members of the research team, depending on the preferences and availability of the primary care center professionals. Approximately 38 intervention face-to-face groups will be conducted, one in each primary care center allocated to the intervention group.

The study will be conducted in Catalonia, Andalusia and the Balearic Islands (Spain).

Effectiveness outcomes include child body mass index (BMI) z-score, parent feeding practices (measured with the Feeding Practices and Structure Questionnaire), child dietary intake (measured with the multiple-pass 24-hour recall), parents' eating behaviours (measured with the Dutch Eating Behavior Questionnaire), child movement (measured with the Movement Behavior Questionnaire), parents' movement behaviours (measured with the Brief Physical Activity Assessment Tool), and child sleep (measured with selected items from the Brief Infant Sleep Questionnaire).

Implementation and feasibility outcomes will be assessed through interviews during and at the end of the intervention, and through attendance sheets and a final satisfaction questionnaire.

This study will provide pioneering insights into the implementation and effectiveness of this intervention within the public healthcare system in Spain. This will allow the potential implementation in the future at a larger scale.

Inclusion Criteria:

• Mothers and/or fathers aged 18 years or older.
• Primary caregivers of a healthy child aged 6 months to 3 years, born at term (≥37 weeks of gestation).
• At least one parent with overweight or obesity (as defined by BMI criteria).
• Families in a situation of vulnerability, defined as meeting ≥2 of the following criteria: (1) Low educational level of at least one parent (≤ lower secondary education or equivalent); (2) Precarious employment situation during the past year, (3) Perceived financial difficulties during the past year; (4) Insecure or unstable housing conditions; (5) Single-parent family or prolonged absence of one of the primary caregivers; (6) Recent immigration (≤10 years) without a stable support network or with language barriers.

Exclusion Criteria:

• Parents of a child aged 6 months to 3 years with low birth weight (<2500 g)
• Parents of a child aged 6 months to 3 years from a multiple birth (e.g., twins, triplets)
• Parents/caregivers of a child aged 6 months to 3 years with congenital conditions, chronic diseases, or severe disabilities that may affect feeding, growth, or participation in the study
• Parents or caregivers with severe physical or mental health conditions that would limit their ability to participate in the intervention or assessments
• Families planning to move out of the study area during the study period