Induced abortion, miscarriage, and the risk of breast cancer—A registry‐based study from Finland

INTRODUCTION: The potential effect of induced abortion and miscarriage on the risk of breast cancer has remained debated and has been a persistent source of misinformation. Many previous studies have been small and based on self-reported data. We assessed the associations of induced abortion and miscarriage with the risk of pre- and postmenopausal breast cancer using high-quality Finnish registry data. MATERIAL AND METHODS: This is a case-control study based on population-based registry data. It includes 31 687 women with breast cancer diagnosed between 1972 and 2021, and their 158 433 female population controls matched by birth year and parity. Data on induced abortions, miscarriages, deliveries, use of postmenopausal hormone therapy, socioeconomic status (SES), and occupation were collected from the Finnish national registries. Multivariate conditional logistic regression analysis was performed. RESULTS: The odds ratio (OR) of breast cancer among women with a history of induced abortion as compared with women with no history of induced abortion was 1.01 (95% confidence interval [CI] 0.92-1.10) in premenopausal (age