Five randomized trials have assessed efficacy of cervical cerclage in prevention of preterm birth (PTB) in singleton gestations with prior PTB and a short cervical length (CL) <25 mm <24 weeks. The largest trial showed significant benefits in reducing PTB <24 weeks, <37 weeks, as well as perinatal mortality. There were trends for benefits for many other outcomes. A patient-level metaanalysis, including all these 5 trials, has shown significant reductions in PTB <35 weeks (by 30%), and in perinatal morbidity and mortality (by 36%). Twenty cerclages are needed to save 1 baby.

These data represent level I data. Tests of heterogeneity and of quality all point to the efficacy of cerclage. Women with prior PTB with asymptomatic short CL <25 mm in singleton gestation with prior PTB may have a condition consistent with cervical insufficiency. Benefit of cerclage does make not only statistical but also clinical sense in this population. It is estimated that >6500 babies per year in the US alone would be saved by this management.

The data prove that women with a singleton gestation and with prior spontaneous PTB (between 16-34 weeks) can be offered screening with transvaginal ultrasound CL starting at 16 weeks. This screening should continue every 2 weeks until 22 6/7 weeks, unless the CL is 25-29 mm, in which case weekly screening can be performed. If CL <25 mm is detected, the risks and benefits of cerclage should be recommended and cerclage should be recommended for reducing the risk of recurrent PTB and of perinatal morbidity and mortality.

Women with prior spontaneous preterm birth should also be offered 17-alpha-hydroxy-progesterone caproate (17P) 250 mg IM weekly from 16 to 36 weeks. The incidence of recurrent preterm birth <28 weeks in these women all taking 17P, if they develop CL <25 mm before 24 weeks, is 15% without and 9% with cerclage placed for the CL <25 mm. 17P is not sufficient in women with prior preterm birth if they develop also a CL <25 mm, which is then an indication for cerclage.