Received: 28 December 2017
Accepted: 30 April 2018
First Online: 10 May 2018
Ethics approval and consent to participate
: Retrospective register based studies do not require ethical approval in Denmark and the study was approved by the Danish Data Protection Agency (Ref. no. 2007–58-0015 /Internal ref. no. GEH-2014-013, I-Suite number: 02731).
: The authors declare that they have no competing interests.
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