Footnotes

• Contributors EEF conceptualised the study, contributed to data collection and analysis and wrote the first draft of this manuscript. ABtP, JD and DW collected data, interpreted the data and critically revised the manuscript. JH contributed to the statistical design, analysed the data and critically reviewed the manuscript. Each named author has read and approved the final draft of this manuscript and agrees to be accountable for all aspects of the work.

• Funding This work was supported by the National Institutes of Health and the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD; grant numbers R03HD086655-01A1 and K23HD084727-01A1).

• Competing interests None declared.

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