- Correspondence to Professor Yvonne Kelly, Department of Epidemiology and Public Health, University College London, 1-19 Torrington Place, London WC1E 6BT, UK,
- Received 4 October 2012
- Revised 9 January 2013
- Accepted 29 May 2013
- Published in print 25 June 2013.
- Published online 12 July 2013.
- Published first 8 July 2013.
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