Received: 25 March 2019
Accepted: 17 January 2020
First Online: 31 January 2020
Ethics approval and consent to participate
: The study was approved by the Regional Committee for Medical and Health Research Ethics in Mid-Norway (reference number 2011/754). Parents provided written informed consent allowing their children to be registered in the CPRN and to link data to the MBRN.
: Not applicable.
: The authors declare that they have no competing interests.