1 National Perinatal Epidemiology Unit, Nuffield Department of Population Health, University of Oxford, Oxford OX3 7LF, UK; Department of Obstetrics, Leiden University Medical Centre, Leiden, Netherlands. Electronic address: t.h.van_den_akker@lumc.nl.
2 Department of Obstetrics, Wilhelmina Children's Hospital Birth Centre, University Medical Centre Utrecht, Utrecht, Netherlands.
3 Department of Obstetrics, Leiden University Medical Centre, Leiden, Netherlands; Athena Institute, VU University, Amsterdam, Netherlands.
4 National Perinatal Epidemiology Unit, Nuffield Department of Population Health, University of Oxford, Oxford OX3 7LF, UK.