1 National Perinatal Epidemiology Unit, Oxford Population Health, Nuffield Department of Population Health, University of Oxford, Oxford, UK.
2 Centre for Statistics in Medicine, Nuffield Department of Orthopaedics, Rheumatology and Musculoskeletal Sciences, University of Oxford, Oxford, UK.
3 National Perinatal Epidemiology Unit, Oxford Population Health, Nuffield Department of Population Health, University of Oxford, Oxford, UK. joris.hemelaar@npeu.ox.ac.uk.