[1] Harvard Univ, Beth Israel Deaconess Med Ctr, Sch Med, Dept Obstet & Gynecol, Boston, MA 02215 USA
[2] Univ London, London Sch Hyg & Trop Med, London, England
[3] Univ London, St Georges Hosp, Sch Med, Dept Community Hlth Sci, London, England
[4] St Georges Health NHS Trust, London, England