Kristiansson 2007.

Methods	Retrospective cohort.
Participants	The cohort consisted of 647,704 women 21 to 43 years of age, born in Sweden, and registered with a first pregnancy (IVF or non‐IVF), from 1981 to 2001 in the nationwide Medical Birth Registry. In this cohort, 8716 women received IVF treatment, of whom 7645 had IVF for their first delivery, whereas 1071 had had a previous delivery without IVF treatment. Women with live birth following pregnancy achieved by IVF treatment in a stimulated cycle, without or with intracytoplasmic sperm injection, were allocated to the IVF group. Women with live birth without such treatment (not in the register of IVF treatment) were allocated to the non‐IVF group. Women with IVF treatment with ovum transfer in a natural cycle or frozen–thawed embryo transfer and women diagnosed with an invasive tumour before the first conception leading to birth were excluded. Women with repeated pregnancies following in vitro fertilization were not taken into account, because the number of cases among women with multiple pregnancies were too few. The categorization of exposure was IVF or non‐IVF where IVF could be multiple IVF pregnancies. Mean follow‐up in the exposed group, 6.2 years; in the non‐exposed, 7.8 years.
Interventions	IVF treatment.
Outcomes	Endometrial cancer. Cases were ascertained by record linkage to the National Cancer Registry in Sweden. Incident cases in cohort, 79; in exposed cohort, 1.
Notes
Risk of bias
Bias	Authors' judgement	Support for judgement
Selection bias (comparability)	High risk	Quote: "Women diagnosed with an invasive tumour before the first conception leading to birth were also excluded." Comment: Outcome not likely to be present at start. Quote: "Women with live birth without such treatment (not in the register of IVF treatment) were allocated to the non‐IVF group". 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	Comment: Analyses were not adjusted for any potential confounding factors.
Performance bias	High risk	Quote: "The Swedish National Board of Health and Welfare is responsible for official statistics about deliveries, assisted reproduction, cancer incidence and causes of death. Data on all deliveries in Sweden from 1973 are reported to the Swedish Medical Birth Registry, which contains individual data collected during pregnancy, delivery and the neonatal period". 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: "Tumour cases were ascertained by record linkage in National Cancer Registry". Comment: Outcome was ascertained by record linkage.
Attrition bias	Low risk	Quote: "Tumour cases were ascertained by record linkage in National Cancer Registry. The overall reporting to the registry is estimated to be 96% of all diagnosed cases". Comment: Follow‐up was expected to be rather complete.
Selective reporting (reporting bias)	High risk	Comment: Standardized Incidence Ratios and Poisson regression results not reported for endometrial cancer.
Other bias	High risk	Quote from Table 1: "Average number of person‐years/woman: 6.4". Comment: Length of follow‐up was considered inadequate (< 10 years). Comment: Non‐RCT study.