[1] Univ Bristol, Sch Social & Community Med, MRC Ctr Causal Anal Translat Epidemiol, Bristol BS8 2BN, Avon, England
[2] Univ Bristol, Sch Social & Community Med, Bristol BS8 2BN, Avon, England
[3] Univ E Anglia, Norwich Med Sch, Norwich NR4 7TJ, Norfolk, England
[4] UCL, Inst Child Hlth, Ctr Paediat Epidemiol & Biostat, London WC1E 6BT, England
[5] UCL, Inst Child Hlth, MRC, Ctr Epidemiol Child Hlth, London WC1E 6BT, England
[6] Univ Glasgow, BHF Glasgow Cardiovasc Res Ctr, Glasgow G12 8QQ, Lanark, Scotland