- Correspondence to Dr Benjamin Saul Raywood Allin, National Perinatal Epidemiology Unit, University of Oxford, Richard Doll Building, Old Road Campus, Headington, Oxford OX3 7LF, UK;
- Received 21 February 2017
- Revised 26 May 2017
- Accepted 1 June 2017
- Published online 7 August 2017.
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