Data were collected at different time points. At the maternity ward, once the adolescent mothers and the maternal grandmothers agreed to participate in the study, they were interviewed individually by the same professionals responsible for the intervention to collect data on sociodemographic characteristics and aspects related to prenatal care, delivery, and previous experience with breastfeeding. Different questionnaires were used for the mothers and for the grandmothers. Data on infant feeding in the first year of life were obtained monthly in the first 6 months, every 2 months between 6 and 12 months of age, always via telephone contact or home visit (whenever telephone contact failed). These data were collected by research assistants who were blind to group allocation. In order to assess the quality of data collection, 5 % of the mothers were drawn and subjected to a second interview with the lead field researcher, containing some key questions of the follow-up questionnaire.