1. Department of Population Health, London School of Hygiene and Tropical Medicine, London, WC1E 7HT, UK
2. Brighton and Sussex University Hospitals Trust, Brighton, BN2 5BE, UK
3. Centre for MARCH (Maternal, Reproductive, Adolescent & Child Health Centre), London School of Hygiene and Tropical Medicine, London, WC1E 7HT, UK
4. International Rescue Committee, New York, NY, 10168-1289, USA
5. Department of Medical Statistics, London School of Hygiene and Tropical Medicine, London, WC1E 7HT, UK
6. National Perinatal Epidemiology Unit, University of Oxford, Oxford, OX3 7LF, UK