[1] RADCLIFFE INFIRM,NATL PERINATAL EPIDEMIOL UNIT,OXFORD OX2 6HE,ENGLAND
[2] ST GEORGES HOSP,SCH MED,DEPT CLIN EPIDEMIOL & SOCIAL MED,LONDON,ENGLAND
[3] UNIV READING,DEPT APPL STAT,READING RG6 2AH,BERKS,ENGLAND
[4] ROYAL FREE HOSP,SCH MED,DEPT CLIN EPIDEMIOL & GEN PRACTICE,LONDON,ENGLAND
[5] MIDDLESEX HOSP,SCH MED,DEPT COMMUNITY MED,LONDON W1,ENGLAND
[6] WESTMINSTER MED SCH,DEPT COMMUNITY MED,LONDON SW1P 2AR,ENGLAND