Abstract: We included five trials, which in total randomised 1577 women, encompassing both singleton and multiple gestations. After excluding singletons, the final analysis included 128 women, of which 122 women had twin gestations, and six women had triplet gestations. Two trials (n = 73 women) assessed history-indicated cerclage, while three trials (n = 55 women) assessed ultrasound-indicated cerclage. The five trials were judged to be of average to above average quality, with three of the trials at unclear risk regarding selection and detection biases. Concerning the primary outcomes, when outcomes for cerclage were pooled together for all indications and compared with no cerclage, there was no statistically significant differences in perinatal deaths (19.2% versus 9.5%; risk ratio (RR) 1.74, 95% confidence intervals (CI) 0.92 to 3.28, five trials, n = 262), serious neonatal morbidity (15.8% versus 13.6%; average RR 0.96, 95% CI 0.13 to 7.10, three trials, n = 116), or composite perinatal death and neonatal morbidity (40.4% versus 20.3%; average RR 1.54, 95% CI 0.58 to 4.11, three trials, n = 116). Among the secondary outcomes, there were no significant differences between the cerclage and the no cerclage groups. To name a few, there were no significant differences among the following: preterm birth less than 34 weeks (average RR 1.16, 95% CI 0.44 to 3.06, four trials, n = 83), preterm birth less than 35 weeks (average RR 1.11, 95% CI 0.58 to 2.14, four trials, n = 83), low birthweight less than 2500 g (average RR 1.10, 95% CI 0.82 to 1.48, four trials, n = 172), very low birthweight less than 1500 g (average RR 1.42, 95% CI 0.52 to 3.85, four trials, n = 172), and respiratory distress syndrome (average RR 1.70, 95% CI 0.15 to 18.77, three trials, n = 116). There were also no significant differences between the cerclage and no cerclage groups when examining caesarean section (elective and emergency) (RR 1.24, 95% CI 0.65 to 2.35, three trials, n = 77) and maternal side-effects (RR 3.92, 95% CI 0.17 to 88.67, one trial, n = 28). Examining the differences between prespecified subgroups, ultrasound-indicated cerclage was associated with an increased risk of low birthweight (average RR 1.39, 95% CI 1.06 to 1.83, Tau² = 0.01, I² = 15%, three trials, n = 98), very low birthweight (average RR 3.31, 95% CI 1.58 to 6.91, Tau² = 0, I² = 0%, three trials, n = 98), and respiratory distress syndrome (average RR 5.07, 95% CI 1.75 to 14.70, Tau² = 0, I² = 0%, three trials, n = 98). However, given the low number of trials, as well as substantial heterogeneity and subgroup differences, these data must be interpreted cautiously. No trials reported on long-term infant neurodevelopmental outcomes. There were no physical exam-indicated cerclages available for comparison among the studies included. This review is based on limited data from five small studies of average to above average quality. For multiple gestations, there is no evidence that cerclage is an effective intervention for preventing preterm births and reducing perinatal deaths or neonatal morbidity.

Plain-language summary: We included five trials, which involved a total of 1577 women, comparing cerclage with no cerclage in women with either singleton and multiple gestations. After excluding singletons, our final analysis included 128 women, of which 122 were pregnant with twins, and six were pregnant with triplets. Cerclage was indicated by obstetric history in two trials (n = 73 women) and transvaginal ultrasound in three trials (n = 55 women). When cerclage was compared with no cerclage in women with multiple gestations, there was no difference in perinatal deaths or neonatal ill health, or preterm birth rates. However, the number of women included in the five studies was insufficient to provide meaningful conclusions. The long-term effect of cerclage on neurodevelopmental outcomes in the surviving infants and maternal infection and side- effects could not be estimated. It was therefore unclear if cerclage for women with multiple pregnancies puts the health of either the mothers or the infants at risk in any way. The five included studies were generally considered to be of average to above average quality, but three of the studies were difficult to assess fully because of missing methodological information. We did not find any studies comparing different indications for cerclage (obstetric history-indicated versus ultrasound-indicated cerclage) or comparing cerclage to another intervention (such as progesterone).