Footnotes

• Twitter @SarahESeaton

• Contributors SES conceived the idea and undertook the analysis, under the supervision of BNM, ESD and KRA. SES wrote the first draft of the paper. NM provided clinical context and medical insight. All authors contributed towards the manuscript, revising it critically for intellectual content. All authors have read and approved this final version of the manuscript.

• Funding This study was funded by Research Trainees Coordinating Centre (grant number: DRF-2013-06-011, NF-SI-0512-10159).

• Competing interests None declared.

• Patient consent Not required.

• Ethics approval Permission was granted to use anonymised data from the NNRD for research purposes (Reference: 14/NW/0349, North West—Lancaster Research Ethics Committee) and agreement was obtained from all neonatal units in England that existed in 2014 to use their data for this project.

• Provenance and peer review Not commissioned; externally peer reviewed.

• Data sharing statement Data used in this study can be obtained from the Neonatal Data Analysis Unit. More information can be found at: http://www.imperial.ac.uk/neonatal-data-analysis-unit/neonatal-data/utilising-the-nnrd/

• Collaborators on behalf of the UK Neonatal Collaborative.