[1] Univ Southampton, Sch Primary Care Populat Sci & Med Educ, Aldermoor Hlth Ctr, Southampton SO16 5SE, England
[2] Haramaya Univ, Sch Nursing & Midwifery, Harar, Ethiopia
[3] Leiden Univ, Dept Obstet & Gynaecol, Med Ctr, Leiden, Netherlands
[4] Univ Oxford, Natl Perinatal Epidemiol Unit, Oxford, England