Owen 2009.

Methods	RCT January 2003 to November 2007. 15 ultrasound clinical centres
Participants	Participants (N = 302) “Healthy multiparous women carrying a singleton gestation who enrolled for prenatal care were screened to identify those with at least 1 prior spontaneous preterm birth between 17 + 0 and 33 + 6 weeks' gestation.” Inclusion criteria “Eligible women consented to serial TV US examinations to measure their cervical length.” “If on any evaluation the cervical length was less than 25 mm, the woman became eligible for randomisation.” Exclusion criteria “fetal anomaly, planned history indicated cerclage for a clinical diagnosis of cervical insufficiency, and clinically significant maternal‐fetal complications (e.g. fetal red cell iso‐immunisation, treated chronic hypertension, insulin‐dependent diabetes) that would increase the risk of an indicated preterm birth and potentially confound the primary study outcome.” Uterine anomalies
Interventions	Cerclage versus “Women in the no‐cerclage group could receive a physical examination indicated cerclage for acute cervical insufficiency diagnosed on clinical examination”.
Outcomes	Primary “birth at < 35 weeks’ project gestational age.” Secondary Rates of birth less than 7 days from randomisation. Perinatal death defined as either a stillbirth or a postnatal death prior to hospital discharge. Preterm birth before 37 weeks
Notes	Additional information and data provided by the first author
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	“Centralized random assignment.” “Randomization in predetermined blocks was stratified by each centre and qualifying cervical length less than 20 mm vs 20‐24 mm.” Stratified randomisation sequence was generated by SAS, permuted in blocks of size 2, 4, and 6.  There was a 1:1 cerclage to no‐cerclage allocation ratio throughout. Early in the study the intent to use progesterone stratification was added
Allocation concealment (selection bias)	Low risk	Centralised randomisation ‐ via the cerclage web site
Blinding of participants and personnel (performance bias) All outcomes	High risk	“Because the cerclage intervention was not masked, managing physicians might infer that the cervical length was less than 25 mm, but they were otherwise masked to the results of the sonographic evaluations except in cases of complete placenta previa, oligohydramnios, or fetal death." Impossible to blind
Blinding of outcome assessment (detection bias) All outcomes	High risk	At delivery, randomisation assignment may or may not have been known.  There was no attempt to blind at delivery
Incomplete outcome data (attrition bias) All outcomes	Low risk	1 participant lost to follow‐up from cerclage group after randomisation. 30/1044 exclusions – 16 ineligible on further review and 14 withdrew from trial. 673/1014 cervical length ≥ 25 mm (23 exclusions: 16 lost or unable to contact; 4 withdrew from trial; 3 became ineligible). 318/1014 cervical length < 25 mm (16 exclusions: 13 declined randomisation; 2 ineligible at randomisation visit; 1 withdrew from trial). 302 randomised. 149 cerclage group: 138/149 received assigned treatment (3 cerclage contraindication; 8 declined to undergo surgery; 1 emergent cerclage revision). 153 no cerclage group: 139/153 received no cerclage (10 received emergent cerclage; 4 received off‐protocol cerclage). 673/1014 cervical length ≥ 25 mm (23 exclusions: 16 lost or unable to contact; 4 withdrew from trial; 3 became ineligible). 318/1014 cervical length < 25 mm (16 exclusions: 13 declined randomisation; 2 ineligible at randomisation visit; 1 withdrew from trial). 302 randomised. 149 cerclage group: 138/149 received assigned treatment (3 cerclage contraindication; 8 declined to undergo surgery; 1 emergent cerclage revision). 153 no cerclage group: 139/153 received no cerclage (10 received emergent cerclage; 4 received off‐protocol cerclage). 149 cerclage group: 138/149 received assigned treatment (3 cerclage contraindication; 8 declined to undergo surgery; 1 emergent cerclage revision). 153 no cerclage group: 139/153 received no cerclage (10 received emergent cerclage; 4 received off‐protocol cerclage). Analysis was intention‐to‐treat
Selective reporting (reporting bias)	Low risk	Study protocol not available, but Cochrane data extraction sheet completed by the authors, so any selective reporting unlikely
Other bias	Low risk	Study was not stopped early. Baseline imbalance: 691 participants declined participation; 1044 met initial criteria and consented