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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.The trial was conducted between June 2010 and August 2013 at seven sites across four countries, namely Burkina Faso, Ghana, Malawi and Zambia (Clin code: NCT00852423).Eligible patients were randomized to one of four treatment arms and followed up weekly until day 63 and then again at delivery.However, correlations varied considerably in between countries (Table 2 & Fig. The mean difference between US and any of the other methods was less than 1 week overall (LMP: 0.34 weeks, SFH: 0.40 weeks, BS: 0.80 weeks) but showed great intercountry variations (Table 2 & Fig. The 95% limits of agreement were considerable (LMP:-7.9 to 8.6 weeks, SFH: − 4.9 to 5.8 weeks, BS: − 3.5 to 5.1 weeks) and again showed great variation in the different countries (Table 2 & Fig. The sensitivity analyses including women ≥24 weeks’ gestation at enrolment into the study showed results consistent with the main analyses for BS with mean difference of 0.66 weeks (Pearson r = 0.27, 95% limit of agreement − 4.1to 5.4, range of averages 27.29–43.21) and higher mean differences for LMP with 1.15 weeks (Pearson r = 0.33; 95% limit of agreement − 7.9 to 10.1, range of averages 25.21–50.14 weeks) and SFH with 0.89 weeks (Pearson r = 0.63, 95% limit of agreement − 4.9 to 6.3, range of averages 22.14–46.85 weeks).Using ultrasound as the reference, 1391 mothers delivered term babies compared to 239 preterm babies ( 24 weeks gestational age at enrolment the sensitivity analyses showed no major difference in trends with regards to agreement between methods for BS but showed higher mean difference in weeks for SFH suggesting an increasing variation .In a sensitivity analysis women ≥24 weeks gestational age at enrolment were included.The level of significance was defined as p ≤ 0.05 and US was considered the reference method.
In order to improve clarity, results of all methods were rounded to the nearest full week for scatterplots .
Gestational age was calculated based on biparietal diameter, abdominal circumference, and femur length  using standard algorithms .
For women in the first trimester of pregnancy, the crown-rump length (CRL) was used to confirm exclusion from the study.
All statistical analyses were done using Stata v14 (Stata Corp, USA).
For the purpose of this analysis, women with a gestational age 24 weeks at enrolment, where the birth date of the baby was not documented, who had twins, miscarriages or stillbirths were excluded.
The mean maternal age at recruitment was 22.3 years (95%CI: 22.1–22.6), with the lowest mean age of 20.6 years (95%CI: 20.2–21.1) in Zambia and the highest with 24.5 years (95%CI: 23.8–25.2) in Ghana (p Results from BS were available for 93.5% (n = 1520) babies, results from SFH for 99.6% (n = 1624) mothers and LMP for 24.8% (n = 404) of enrolled women.