Reigstad 2015.

Methods	Retrospective cohort.
Participants	The cohort consisted of 806,248 women, born in Norway, and registered with a first pregnancy (IVF or non‐IVF), from 1984 to 2010 in the Medical Birth Registry of Norway. In this cohort, 16,525 women gave birth to a child following assisted reproductive technology (ART). Women who had at least one pregnancy initiated by ART (IVF, ICSI, a combination of the two or any other kind of treatment) were classified as ART women, and women who had no registered ART pregnancies were classified as non‐ART. Subjects who delivered after emigrating from Norway, and those with a cancer prior to the start of follow‐up were excluded. The categorization of exposure was ART or non‐ART where ART could be multiple ART pregnancies. Median follow‐up in the exposed group, 7.3 years; in the non‐exposed, 16.0 years.
Interventions	Assisted reproductive technology (ART)
Outcomes	Endometrial cancer. Cases were ascertained by record linkage to the Cancer Registry of Norway (CRN). Incident cases in cohort, 636; in exposed cohort, 5.
Notes
Risk of bias
Bias	Authors' judgement	Support for judgement
Selection bias (comparability)	High risk	Quote: "After removal of subjects who delivered after emigrating from Norway, and those with a cancer prior to the start of follow‐up". Comment: Outcome not likely to be present at start. Quote: "Women who had no registered ART pregnancies were classified as non‐ART". Comment: Non‐exposed drawn from the same population as the exposed cohort. Comment: No comparability on cause of subfertility, diabetes mellitus, polycystic ovary syndrome (PCOS), and obesity was ensured.
Selection bias (confounding)	High risk	Quote: "To adjust for the age difference between ART women and non‐ART women, we used the age of study subjects as the time‐scale. Confounder adjustment was made for age at start of follow‐up, calendar period, region of residence in Norway and parity as a time‐varying covariate, as previously described". Comment: Analyses inadequately controlling for potential confounding factors.
Performance bias	High risk	Quote: "All deliveries (from Week 22) in Norway have been recorded in the MBRN since its establishment in 1967. The reporting of data on ART pregnancies started in 1984 (the year the first baby was born after IVF in Norway). For each child born, the following data were extracted from the MBRN: date of birth of mother and child, parity, present region of residence, exposure to ART, the specific method of ART". Comment: Exposure to ovary‐stimulation drugs was ascertained by a secure source, namely, medical records. Blindness regarding the allocated interventions not guaranteed.
Detection bias	Low risk	Quote: "All women with at least one diagnosis of invasive cancer during the period 1 January 1953 through 31 December 2010 were identified through linkage of the MBRN data to the Cancer Registry of Norway (CRN) using each woman’s unique personal identification number". Comment: Outcome was ascertained by record linkage.
Attrition bias	Low risk	Quote: "The completeness of the CRN enabled accurate ascertainment of cancer, with negligible losses to follow‐up". Comment: Follow‐up was expected to be rather complete.
Selective reporting (reporting bias)	High risk	Results of stratified analyses were not presented.
Other bias	High risk	Quote from Table 1: "Follow‐up time, person years (median): 15.9". Comment: Length of follow‐up was considered adequate (> 10 years). Comment: Non‐RCT study.