[1] Univ Cambridge, Dept Paediat, MRC, Epidemiol Unit, Cambridge, England
[2] Inst Child Hlth, MRC, Childrens Nutr Res Ctr, London, England
[3] Ctr Dis Control & Prevent, Natl Ctr Environm Hlth, Atlanta, GA USA
[4] Univ Bristol, Dept Community Based Med, Bristol, Avon, England
[5] Univ Cambridge, Dept Paediat, Cambridge, England