Secondary outcomes
Secondary outcomes included PTB less than 28, 32, 34, and 35 weeks. Maternal secondary outcomes examined were rates of preterm premature rupture of the membranes
(PPROM), chorioamnionitis (clinical or laboratory diagnosis), cervical laceration and stenosis, caesarean section delivery (excluding planned or elective), number of days between cerclage and delivery, intraoperative membrane rupture at time of cerclage, cases requiring repeat cerclage and difference in cervical length before and after cerclage. Neonatal outcomes included birthweight, Apgar score <7, and neonatal mortality, which included mortality up to one month post-delivery or neonatal survival (this figure was then inverted to mortality). Thorough definitions of these outcomes are referenced in the protocol paper(9).
Data synthesis
A meta-analysis was performed by pooling studies together using RevMan(24) and Covidence software(20). The heterogeneity of data was examined using forest plots and quantified throughout calculation of the I² value. An I² of greater than or equal to 50% was used to indicate substantial heterogeneity and a random-effects model was used. For all I2 less than 50%, a fixed effects model was used. Outcomes with less than five studies were analysed using a fixed effects model(25). For reporting consistency between outcomes, we made the McDonald intervention the reference set for all analyses, standardising the direction of effect across all primary and secondary outcomes.
 
Measures of treatment effect 
Where applicable, trial data were combined and reported using meta-analyses using the standard estimation of: 1) risk ratio (RR) and 95% confidence intervals (CI) for dichotomous outcome variables, and 2) mean differences (MDs) or standardised mean differences (SMDs) and 95% CIs for continuous outcome variables.
Sensitivity Analysis
Sensitivity analysis was conducted on the primary outcomes – PTB less than 37 weeks gestation. This was performed by removing studies with an overall high risk of bias to examine their impact on the effect estimate.