2.1.1 Literature search
The Cochrane Library, PubMed, EMbase, WanFang Data, CNKI, VIP Data and CBM Data were electronically searched to collect randomized controlled trials (RCTs) and non-randomized controlled trials (non-RCTs, including cohort studies) comparing cervical pessary and transvaginal cervical cerclage in preventing spontaneous preterm birth. According to the PRISMA Guidelines and the Cochrane Handbook for Systematic Reviews of Interventions[6-7], the literature search was conducted using MeSH terms combined with entry terms (e.g., pessaries, pessary, cervical cerclage, cerclage, cervical, premature birth, preterm birth, etc.). Taking PubMed for example, the detailed search strategy is provided in Box 1-1. All electronic databases were searched from their inception to August 31, 2020.