Pregnancy dating criteria

Inter-country and inter-method comparisons were done using the Kruskal-Wallis method; correlation coefficients were calculated using Pearson’s rho (r), Bland Altman plots were used to calculate mean differences in findings and 95% limit of agreements (Lo A).

The level of significance was defined as p ≤ 0.05 and US was considered the reference method.

Comprehensive quality assurance and quality control (QA/QC) systems were put in place to ensure the quality and reliability of measurements, and the inter-site comparability of US measurements.

This included centrally purchased equipment, a standard operating procedure (SOP) which was applicable and mandatory across all sites (Additional file 1), two specifically dedicated staff per site to carry out all US measurements and central training before study start.

At enrolment into the study, patients were asked about the date of their LMP.

LMP was defined as the date of the first day of the last menstruation.