1 National Perinatal Epidemiology Unit, Nuffield Department of Population Health, University of Oxford, Old Rd. Campus, Oxford, OX3 7LF, United Kingdom. Electronic address: marian.knight@npeu.ox.ac.uk.
2 Department of Health Sciences, College of Life Sciences, University of Leicester, Leicester, United Kingdom.
3 National Perinatal Epidemiology Unit, Nuffield Department of Population Health, University of Oxford, Oxford, United Kingdom.