Participants: 
The review considered all studies that included pregnant women undergoing McDonald or Shirodkar cervical cerclage for prevention of PTB. Studies were excluded if they included women with multiple gestation pregnancies.
Intervention :
Studies which compared the McDonald and Shirodkar techniques of cervical cerclage as a prophylactic procedure.
McDonald Cerclage
In the McDonald approach a suture is placed around the cervix in purse-string fashion and securely tied anteriorly. The McDonald approach requires no dissection into para-cervical tissues(13, 14).
Shirodkar Cerclage
The Shirodkar technique involves a transverse anterior colpotomy, dissection of the bladder up to the internal cervical os and a posterior colpotomy with dissection of areola and peritoneum upwards to the internal os. The suture is placed subcutaneously and the knot tied in the posterior defect and buried under the vaginal epithelium(13, 15, 16). Later modifications do not require a posterior colpotomy and place the knot exterior to the vaginal mucosal for ease of removal(17). Whilst technically more challenging, the rationale of this technique is to allow more proximal placement of cerclage closer to the internal os. A number of other modifications have been reported which simplify the Shirodkar technique by utilising a clamp on the paracervical tissues for more accurate suture needle placement(18) or avoiding a posterior colpotomy and suture burial(17). For the purpose of this study both the original technique described by Shirodkar and the modified techniques were included.
Types of studies:
This review accepted randomised control trials, pseudo-randomised control trials, non-randomised experimental control trials and cohort studies. All papers included had to compare the co-interventions, McDonald and Shirodkar cerclage.
Search Strategy:
Six electronic bibliographic databases were searched for eligible, peer-reviewed literature: Medline (Ovid), EMBASE (Ovid), PsycINFO (Ovid), Scopus, CINAHL (EBSCOhost), and Cochrane Library (Wiley). Reference lists of included studies were screened and references in academic textbooks were also reviewed. Where studies were unable to be sourced contact was attempted with the corresponding author. A more detailed database search strategy is described in Appendix S1.
Data collection and analysis
Study Selection
The titles and abstracts were reviewed using Endnote(19) and Covidence(20). Studies that did not meet the criteria based on abstracts were excluded (authors A-M.A and L.M.) and full texts of remaining articles were sourced and screened (A-M.A and R.D.). No language restriction was set, all non-English included studies were translated. Included studies were critically appraised (by L.M and A.I.) and data extracted using a standardised electronic form (by R.D. and K.P.W.). At all levels of screening, any discrepancies were moderated by a third senior reviewer (C.E.P.).
Assessment of risk of bias
To facilitate the assessment of possible risk of bias for each study, two independent reviewers (A.I and L.M) assessed each paper using the Cochrane Collaboration tool for assessing the risk of bias (ROBINS-I & RoB 2)(21, 22) for non-randomised and randomised studies respectively(12).
Cochrane GRADE Assessment
Quality of evidence for our primary outcome was judged using the GRADE tool by two independent reviewers (A-M.A. and K.P.W.)(23).