Noehr 2009a.

Methods	Retrospective cohort study Comparison groups: A) External B) Women with biopsy but no treatment Both were unmatched but had regression analysis for age, year of delivery, smoking during pregnancy and marital status during pregnancy Information source ‐ Medical Birth Registry, National Patient Registry, Danish Registry of Pathology, Danish IVF Registry
Participants	Treated group ‐ 10,207 deliveries of women who had undergone LLETZ or ablation (LLETZ = 8180; Ablation = 2027) during 1997 to 2005 and had a subsequent singleton delivery (21 to 45 weeks of gestation) during 1997 to 2005. Exclusion criteria: women with previous CKC; deliveries with medical induction before 37 completed weeks of gestation; infants delivered by CS performed before 37 completed weeks of gestation; deliveries subsequent to a collum amputation. Untreated group ‐ A) 510,841 singleton deliveries (21 to 45 weeks of gestation during 1997 to 2005) of women without previous cervical procedure (treatment or biopsy) B) 31,630 singleton deliveries (21 to 45 weeks of gestation during 1997 to 2005) of women without previous treatment but with previous biopsy.
Interventions	LLETZ; Ablation NOS
Outcomes	sPTB (< 37 weeks); sPTB (< 37 weeks) (D ≤ 12 mm); sPTB (< 37 weeks) (D = 13 mm to 15mm); sPTB (< 37 weeks) (D = 16 mm to 19 mm); sPTB (< 37 weeks) (D ≥ 20 mm); sPTB (< 37 weeks) (single cone); sPTB (< 37 weeks) (repeat cones); sPTB (< 37 weeks) (singleton pregnancies); sPTB (< 32 weeks); sPTB (< 28 weeks)
Notes	Of the 552,678 deliveries in the study, 8180 deliveries were subsequent to LLETZ (1 LLETZ: 7907; 2 LLETZ: 255; ≥ 3 LLETZ: 18). Of these 8180 deliveries, 162 were subsequent to both LLETZ and ablation. The addition of the number of deliveries subsequent to biopsy, ablation, LLETZ and no procedure (table 3 of the article) gives the total number of pregnancies (552,678), thus we concluded that the authors subtracted these 162 deliveries from the ablation group and left them only in the LLETZ group. In this way, we made sure that we will not make a duplicate extraction of the same deliveries. The Nordic Classification of Surgical Procedures has the same code for LLETZ and LC and the authors were not able to separate LLETZ from LC. However, LC has become rare in Denmark since the introduction of LLETZ and the authors included the pregnancies with this code in the LLETZ group. In our meta‐analysis, we made the same. In a second article, Noehr and colleagues 2009 investigated the association between cone depth of the LLETZ and the subsequent risk of sPTB (on the same singleton deliveries as above). Deliveries after LLETZ with no information on cone depth (n = 4302) and deliveries after two or more LLETZ (n = 273) were not included in the cone depth analysis. Nohr and colleagues 2007 is an overlapping study, but it was impossible to extract only the data that was not included in Noehr and colleagues 2009 and we excluded this study.
Risk of bias
Bias	Authors' judgement	Support for judgement
Incomplete outcome data (attrition bias) All outcomes	Low risk	Information was obtained from national registries
Selective reporting (reporting bias)	Low risk	No reporting bias is obvious
Other bias	Unclear risk	The Nordic Classification of Surgical Procedures has the same code for LLETZ and LC and the authors were not able to separate LLETZ from LC. However, LC has become rare in Denmark since the introduction of LLETZ and the authors included all these pregnancies in the LLETZ group.
Relevant assignment described?	Low risk	Yes, treatment performed on clinical grounds
Representative intervention group?	Low risk	All eligible for the study Danish women delivering during 1997 to 2005 (a population‐based study)
Representative comparison group?	Low risk	The 2 comparison groups were drawn from the same source as the treated group
Comparability of treatment groups?	Low risk	Regression analysis for age, year of delivery, smoking during pregnancy and marital status during pregnancy