Risk of bias and quality of evidence
Table 2A and 2B show the level of risk in each of the domains of bias.
Risk of bias for individual studies was determined using the Cochrane ROBINS-I for cohort studies and RoB-2 for the randomised control trials(21, 22).
In examining bias risk, it was found that more dated trials tended to have a higher risk of bias whereas more recent ones appeared to show more favourable risk of bias. This is likely due to improved trial design and reporting. Thirteen papers (76.5%) were judged to be at serious risk of bias, six (35.3%) at moderate risk, and two (11.7%) at low risk. The quality of evidence for the primary outcome, birth before 37 weeks, was assessed using GRADE and was ranked as low quality.