Klaritsch 2006.

Methods	Retrospective cohort study Comparison group: External ‐ no matching, no regression analysis Information source ‐ Hospital records
Participants	Treated group ‐ 76 singleton deliveries of 65 women who delivered at the Department of Obstetrics & Gynaecology, Medical University of Graz, Austria, between 1992 to 2002 and had previously undergone CKC at the same hospital or other hospital. Exclusion criteria: Women with LLETZ, repeated conisation, previous PTB or multiple gestations. No woman had undergone CKC during pregnancy. Untreated group ‐ all singleton deliveries (29711) of the women who delivered at the Department of Obstetrics & Gynaecology, Medical University of Graz, Austria, between 1992 to 2002 and did not have a history of cervical conisation
Interventions	CKC
Outcomes	PTB (< 37 weeks); PTB (< 37 weeks) (single cone); PTB (< 37 weeks) (singleton pregnancies); PTB (< 34 weeks); pPROM; CS; chorioamnionitis; LBW (< 2500 g); perinatal mortality
Notes	There were 29,809 singleton deliveries at the University Hospital of Graz between 1992 and 2002. 98 deliveries of 86 women with history of conisation were identified. The researchers excluded 21 women and their 22 deliveries. 16 of them had undergone LLETZ, 2 had had more than one conisation, 1 had had previous PTB, and 2 had multiple gestations. 65 women with a total of 76 deliveries had undergone CKC and were included in the conisation group. For controls the researchers took the remaining 29,711 singleton deliveries in the study period. 53 women had undergone CKC at the University Hospital of Graz, 12 at other hospitals. No woman had undergone CKC during pregnancy.
Risk of bias
Bias	Authors' judgement	Support for judgement
Incomplete outcome data (attrition bias) All outcomes	Low risk	Complete data in the control group; a small percentage of incomplete data for most outcomes in the untreated group
Selective reporting (reporting bias)	Low risk	No reporting bias is obvious
Other bias	Low risk	No other obvious source of bias
Relevant assignment described?	Low risk	Yes, treatment performed on clinical grounds
Representative intervention group?	Low risk	All eligible for the study women delivering in a single hospital between 1992 to 2002
Representative comparison group?	Low risk	The untreated group was drawn from the same source as the treated group
Comparability of treatment groups?	High risk	Neither matching nor regression analysis; the median age at delivery was 30 years (range 21 to 43) in the treated group vs 28 years (range 14 to 61) in the untreated group; 73.7% multigravidas in the treated group vs 49.9% multigravidas in the untreated group; no significant difference in smoking habits