The purpose of this study was to determine the efficacy of cerclage and bed rest versus bed rest only for the prevention of preterm birth in women with a short cervix found on transvaginal ultrasound examination. Women with ≥ 1 of high risk factors for preterm birth (≥ 1 preterm birth at < 35 weeks of gestation, ≥ 2 curettages, diethylstilbestrol exposure, cone biopsy, M¨ ullerian anomaly, or twin gestation) were screened with transvaginal ultrasonography of the cervix every 2 weeks from 14 weeks of gestation to 23 weeks 6 days of gestation. Enrollment was offered to both asymptomatic women who were at high risk and who were identified to have short cervix (< 25 mm) or significant funneling ( >25% ) and nonscreened women who were at low risk and who were identified incidentally. The women who gave written consent were assigned randomly to receive either McDonald cerclage or bed rest only. Both groups received similar counseling and treatment. Primary outcome was preterm birth at <35 weeks of gestation. Sixty one women were assigned randomly. Forty seven pregnancies (77% ) were high risk singleton gestations. Thirty one women (51% ) were allocated to cerclage, and 30 women (49% ) were allocated to bed rest. There were no differences between the groups in demographic characteristics, risk factors, and cervical variables. Preterm birth at < 35 weeks of gestation occurred in 14 women (45% ) in the cerclage group and in 14 women (47% ) in the bed rest group (relative risk, 0.94; 95% CI, 0.34- 2.58). There was no difference in any obstetric or neonatal outcomes. A subanalysis of singleton pregnancies with previous preterm birth at < 35 weeks of gestation and a short cervix of < 25 mm (n = 31 women) also revealed no significant difference in recurrent preterm birth at < 35 weeks of gestation (40% vs 56% ; relative risk, 0.52; 95% CI, 0.12- 2.17). Cerclage did not prevent preterm birth in women with a short cervix. These results should be confirmed by larger trials. The purpose of this study was to determine the efficacy of cerclage and bed rest versus bed rest only for the prevention of preterm birth in women with a short cervix found on transvaginal ultrasound examination. Women with ≥ 1 of high risk factors for preterm birth (≥ 1 preterm birth at <35 weeks of gestation, ≥ 2 curettages, diethylstilbestrol exposure, cone biopsy, M¨ ullerian anomaly, or twin gestation) were screened with transvaginal ultrasonography of the cervix every 2 weeks from 14 weeks of gestation to 23 weeks 6 days of gestation. Enrollment was offered to both asymptomatic women who were at high risk and who were identified to have short cervix (<25 mm) or significant funneling (> 25%) and nonscreened women who were at low risk and who were identified incidentally. The women who gave written consent were assigned randomly to receive either McDonald cerclage or bed rest only. Both groups received similar counseling and treatment. Primary outcome was preterm bi Forty seven pregnancies (77%) were high risk singleton gestations. Thirty one women (51%) were allocated to cerclage, and 30 women (49%) were allocated to There were no differences between the groups in demographic characteristics, risk factors, and cervical variables. Preterm birth at <35 weeks of gestation occurred in 14 women (45%) in the cerclage group and in 14 women (47%) in There was no difference in any obstetric or neonatal outcomes. A subanalysis of singleton pregnancies with previous preterm birth at <35 weeks of gestation and a short cervix of <25 mm (n = 31 women) also revealed no significant difference in recurrent preterm birth at <35 weeks of gestation (40% vs 56%; relative risk, 0.52; 95% CI, 0.12-2.17) birth in women with a short cervix. These results should be confirmed by larger trials.