OBJECTIVE: To estimate if cerclage prevents preterm birth and perinatal mortality and morbidity in women with previous preterm birth, singleton gestation, and short cervical length in a meta-analysis of randomized trials. DATA SOURCES: MEDLINE, PUBMED, EMBASE, and the Cochrane Library were searched using the terms "cerclage," "short cervix," "ultrasound," and "randomized trial." METHODS OF STUDY SELECTION: We included randomized trials of cerclage in women with short cervical length on transvaginal ultrasonography, limiting the analysis to women with previous spontaneous preterm birth and singleton gestation. TABULATION, INTEGRATION, AND RESULTS: Patient-level data abstraction and analysis were accomplished by two independent investigators. Five trials met inclusion criteria. In women with a singleton gestation, previous spontaneous preterm birth, and cervical length less than 25 mm before 24 weeks of gestation, preterm birth before 35 weeks of gestation was 28.4% (71/250) in the cerclage compared with 41.3% (105/254) in the no cerclage groups (relative risk 0.70, 95% confidence interval 0.55-0.89). Cerclage also significantly reduced preterm birth before 37, 32, 28, and 24 weeks of gestation. Composite perinatal mortality and morbidity were significantly reduced (15.6% in cerclage compared with 24.8% in no cerclage groups; relative risk 0.64, 95% confidence interval 0.45-0.91). CONCLUSION: In women with previous spontaneous preterm birth, singleton gestation, and cervical length less than 25 mm, cerclage significantly prevents preterm birth and composite perinatal mortality and morbidity. © 2011 The American College of Obstetricians and Gynecologists.