The QLSCD cohort study recruited singletons born in Quebec, Canada between October 1997 and July 1998, excluding certain territories, totalling 94.5% of the target population. A stratified random sample of 2,223 families participated in the first data collection at 5 months of age, with 1,245 participants retained at age 20, revealing higher dropout rates among males and socioeconomically disadvantaged families, potentially linked to maternal alcohol use and postnatal depressive symptoms. Children were included if they were born between 24 and 42 weeks of gestation and if their mothers could speak French or English.