1. Department of Medical Statistics, Manchester University NHS Foundation Trust, Manchester, UK
2. Centre for Biostatistics, Division of Population Health, Health Services Research and Primary Care, University of Manchester, Manchester, UK
3. Department of Obstetrics and Gynaecology, University of Auckland, New Zealand
4. Medical Research Council Clinical Trials Unit, University College London, London, UK
5. Department of Reproductive Medicine, Manchester University NHS Foundation Trust, Manchester Academic Health Science Centre, Manchester, UK
6. Maternal and Fetal Health Research Centre, Faculty of Biology, Medicine and Health, University of Manchester, Manchester Academic Health Sciences Centre, Manchester, UK