Affiliation: | 1. Academic Unit of Obstetrics and Gynaecology, Manchester Academic Health Science Centre, University of Manchester, School of Cancer and Enabling Sciences, Manchester M13 OJH, UK;2. Biostatistics, Health Sciences – Methodology, Manchester Academic Health Science Centre, School of Community Based Medicine, University of Manchester, Manchester M13 9PT, UK;3. Public Health Department, Bury Primary Care Trust, Bury M45 7TA, UK;4. Clinical Epidemiology and Public Health Unit, University of Manchester, Manchester M13 9PT, UK;5. Greater Manchester Health Protection Unit, Eccles M30 0NJ, UK |