1. Keele University, Room 75, Keele Hall, Newcastle, Staffordshire ST5 5BG, England,
2. Policy Research Department, National Childbirth Trust, Alexandra House, Oldham Terrace, London W3 6NH, England
3. School of Maternal and Child Health, Faculty of Health and Social Care, University of West England Bristol, Blackberry Hill, Stapleton, Bristol BS16 1DD, England,
4. Mathematics Department, Keele University, Mackay Building, Newcastle, Staffordshire ST5 5BG, England