[1] Univ Leicester, Dept Obstet & Gynaecol, Leicester, Leics, England
[2] Univ Leicester, Dept Epidemiol & Publ Hlth, Leicester, Leics, England
[3] Queen Charlottes & Chelsea Hosp, Dept Obstet & Gynaecol, London W6 0XG, England
[4] St Thomas Hosp, Dept Obstet & Gynaecol, London, England
[5] Natl Univ Ireland Univ Coll Cork, Dept Epidemiol & Publ Hlth, Cork, Ireland