Ehsanipoor 2014.

Methods	Retrospective cohort study Comparison group: External ‐ unmatched; regression analysis for age, parity, race, history of PTB, history of tobacco use, history of drug use and chorionicity Information source ‐ maternal prenatal records and impatient hospital charts from two community hospitals in California
Participants	Treated group ‐ 110 women who had a twin pregnancy (≥ 24 weeks of gestation) at two community hospitals in California during 1998 to 2005 and had previously undergone treatment for CIN (CKC = 10, LLETZ = 36, LA/CT = 64) Exclusion:women with colposcopy or biopsy only, pregnancies with major fetal anomalies or intrauterine death, multi‐fetal pregnancy reduction, indicated delivery prior to 34 weeks, twin–twin transfusion syndrome, or cerclage placement Untreated group ‐ 766 women who had a twin pregnancy (≥ 24 weeks of gestation) at two community hospitals in California during 1998 to 2005 with no history of cervical procedures
Interventions	CKC; LLETZ; Ablation NOS (LA, CT)
Outcomes	PTB (< 37 weeks) (multiple pregnancies); PTB (< 34 weeks) (multiple pregnancies); PTB (< 28 weeks) (multiple pregnancies)
Notes	No woman had more than one twin delivery during the time period specified. If a participant had undergone both an ablative and excisional procedure, she was included in the excisional group. A total of 110 (12.6%) women had undergone a prior procedure for cervical dysplasia. This included 10 with a CKC, 36 with a LEEP, 59 with cryotherapy and 5 had undergone CO2 laser ablation. One of the participants with a CKC also had cryotherapy. One participant had undergone cryotherapy twice and none of the women had more than one excisional procedure.
Risk of bias
Bias	Authors' judgement	Support for judgement
Incomplete outcome data (attrition bias) All outcomes	Low risk	Information was obtained from hospital records
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 having a twin pregnancy in two hospitals in California during 1998 to 2005
Representative comparison group?	Low risk	The untreated group was drawn from the same source as the treated group
Comparability of treatment groups?	Low risk	Regression analysis for age, parity, race, history of PTB, history of tobacco use, history of drug use and chorionicity