3D investigates adverse birth outcomes such as intrauterine growth restriction (IUGR) and preterm birth (PTB) and their intrauterine determinants, as well as neurodevelopmental, academic, social and mental health outcomes in offspring. The study recruited participants aged at least 18 years who could speak English or French, were between 8 and 14 weeks pregnant, and intended to deliver in one of nine collaborating hospitals in Montreal, Quebec City and Sherbrooke in Quebec, Canada. At baseline, over 2,300 women were enrolled in the study. The partner and child were also recruited into the study, forming a trio. At baseline, over 2,300 triads were in the study, resulting in over 7,000 participants. Between 2017 and 2025, two follow-up studies have been conducted on a subset of the child participants. 3D-Transition followed 939 participants as they transitioned to kindergarten between 2017 and 2022. 3D-Transition-Ado began in 2023 and follows 608 participants as they transition to puberty and adolescence.