Footnotes

• Contributors PC helped collect data, analyse the data and edit the manuscript. ML created the protocol and study documents, obtained the relevant approvals for the project, collected data, analysed data and wrote the manuscript. ML had full access to all of the data in the study and can take responsibility for the integrity of the data and the accuracy of the data analysis. GH is the chief investigator for the study and helped create the study—including protocol/study documents, ethical approval, sites for data collection and data analysis. SC advised on the design and analysis of the study, and critically revised the manuscript and contributed to the final draft. DH conceived the study, and critically revised the manuscript and contributed to the final draft.

• Competing interests None declared.

• Ethics approval The West Midlands – Coventry and Warwickshire Research Ethics Committee (ID: 16/WM/0158).

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

• Presented at This research was presented orally at the Royal College of Obstetrics and Gynaecology World Congress, March 2017.