We also explored associations between known risk factors and outcomes in a representative birth cohort and a selected sub-study. We used ALSPAC as the birth cohort. Initially, 14 541 pregnant women who were expected to give birth between 1 April 1991 and 31 December 1992 were recruited into the study in the South West region of England.19 The study website contains details of all data available through a fully searchable data dictionary: [http://www.bris.ac.uk/alspac/researchers/data-access/data-dictionary/]. Ethics approval for the study was obtained from the ALSPAC Ethics and Law Committee and the local research ethics committees. We also used the Accessible Resource for Integrated Epigenomics Studies (ARIES), a sub-study of ALSPAC where a sub-set of 1018 mother-offspring pairs were selected based on availability of DNA samples at two time points for the mother (at an antenatal clinic and at a follow-up clinic when their offspring were mean age 15.5 years) and three time points for the offspring (at birth, childhood and adolescence.2 We investigated the association between a genetic risk score for smoking (ever vs never) and maternal education in ALSPAC, and in
