1. Department of Population Health, Faculty of Epidemiology and Population Health, London School of Hygiene and Tropical Medicine, Keppel Street, London WC1E 7HT, UK
2. Maternal and Child Wellbeing Unit, African Population and Health Research Center, Nairobi, Kenya
3. Population Health Sciences Institute, Faculty of Medical Sciences, Newcastle University, Newcastle upon Tyne, Tyne and Wear, UK
4. Epidemiology and Public Health, Institute of Global Health, University College London, London, UK