WCHADS aims to identify social, emotional and biological risks and processes involved in the development of child and adolescent emotional and behavioural problems. Between 2007 and 2008, the study recruited over 1,200 first-time pregnant women aged 18 years and above at their 20-week scan appointment in Wirral, Merseyside (England, United Kingdom). Participants have been assessed at 20 weeks of pregnancy, birth, 9-12 weeks old, 14 months old, 3.5 years, 4.5-5 years, 7 years, 9 years, 12 years, 13 years, and 15-16 years, with plans to continue following families as the young people transition towards adult life. A subgroup of 316 mothers and children (the “intensive sample”) has been assessed at additional time points (4 weeks, 7 months, 14 months, 2.5 years, 4.5 - 5 years, 7 years and 9 years).