1Centre for Occupational and Environmental Health, School of Community-Based Medicine, Faculty of Medical and Human Sciences, University of Manchester, Oxford Rd, Manchester M13 9PL, UK
2Department of Reproductive Medicine, Manchester Academic Health Sciences Centre, St Mary's Hospital, Hathersage Rd, Manchester M13 0JH, UK
3Health Methodology Research Group, School of Community-Based Medicine, Faculty of Medical and Human Sciences, University of Manchester, Oxford Rd, Manchester M13 9PL, UK