[1] Univ Oxford, Nuffield Dept Populat Hlth, Natl Perinatal Epidemiol Unit, Old Rd Campus, Oxford OX3 7LF, England
[2] Queen Mary Univ London, Barts & London Sch Med & Dent, Ctr Environm & Prevent Med, London, England
[3] UCL, Inst Womens Hlth, London, England