A Cochrane review (abstract , review ) included 6 studies with a total of 2175 women. Prophylactic cerclage was compared with no cerclage in four trials. There was no overall reduction in pregnancy loss and preterm delivery rates, although a small reduction in births under 33 weeks' gestation was seen in the largest trial (relative risks 0.75, 95% confidence interval 0.58 to 0.98). Cervical cerclage was associated with mild pyrexia, increased use of tocolytic therapy and hospital admissions but no serious morbidity. Two trials examined the role of therapeutic cerclage when ultrasound examination revealed short cervix. Pooled results failed to show a reduction in total pregnancy loss, early pregnancy loss or preterm delivery before 28 and 34 weeks in women assigned to cervical cerclage.
Comment: The quality of evidence is downgraded by indirectness (differences in studied patients).