The study originally aimed to investigate early-life exposures and their effects on health outcomes, with special attention to physical activity and social inequalities. It has expanded to include follow-ups to monitor life-course determinants of health as children move into middle childhood, investigate psychosocial development and violence, and understand the impacts of the COVID-19 pandemic. The 2015 cohort is the fourth in a series of similar cohort studies in Pelotas, Brazil, which included children born in 1982, 1993 and 2004. Participants were children born between 1 January and 31 December 2015 to women living in the city of Pelotas. Mothers completed baseline assessment during pregnancy, and children were assessed at birth in 2015. Children were followed up in 2017, 2019, 2020, 2021 to 2022 and 2025.