Risk of breast cancer following fertility treatment—A registry based cohort study of parous women in Norway

Despite increasing numbers of women availing themselves of assisted reproductive technology (ART), effects on cancer risk remain unresolved. Given hormonal exposures, breast cancer risk is of particular concern. The aim of this study is to investigate breast cancer risk amongst women giving birth following ART as compared to that amongst women who gave birth without ART. Data on all women who gave birth in Norway with or without ART, between 1984 and 2010 were obtained from the Medical Birth Registry of Norway (MBRN). 808,834 women eligible for study were linked to the Cancer Registry of Norway. Cox proportional models computed hazard ratios (HR) and 95% confidence intervals (CI) of breast cancer between the two groups, adjusting for age, parity, age at first birth, calendar period and region of residence. In total, 8,037 women were diagnosed with breast cancer during the study period, 138 ART women and 7,899 unexposed. Total follow‐up time was 12,401,121 person‐years (median 16.0); median age at entry was 32.5 years (range18.6–49.9) for ART women and 26.3 (range 10.5–54.6) for unexposed. Women exposed to ART had an elevated risk of breast cancer (adjusted HR 1.20, 95% CI 1.01–1.42). Subgroup analyses gave an HR of 1.30 (95% CI 1.07–1.57) for women treated with IVF and 1.35 (95 % CI 1.07–1.71) for women with follow‐up >10 years, compared with controls. Our findings of increased risk in the study population warrant continued monitoring of women treated with ART as this population advances into more typical cancer age ranges. What's new? Assisted reproductive technology (ART) typically involves the use of hormonal therapy, suggesting possible effects on cancer risk for infertile women. But whether ART is associated with increased or decreased risk of breast cancer in particular is unclear. Here, based on data from the Cancer Registry of Norway and the Medical Birth Registry of Norway, women who give birth following ART are at increased risk of breast cancer compared with women who give birth without ART. The rise in risk was greatest amongst women who were followed for more than 10 years and women who used in‐vitro fertilization specifically.