Induced abortion and miscarriage were not associated with an increased risk of either pre- or postmenopausal breast cancer in a large Finnish registry-based case–control study, adding high-quality evidence to a topic that has remained a persistent source of public misinformation.
Researchers analyzed nationwide registry data from Finland, including 31,687 women diagnosed with breast cancer between 1972 and 2021 and 158,433 matched population controls.
Unlike earlier studies that relied on self-reported pregnancy histories, the current analysis used mandatory national registries capturing induced abortions since 1950 and miscarriages treated by physicians since 1969. Researchers also adjusted for socioeconomic status and occupational physical activity, and they included postmenopausal hormone therapy use as a covariate in analyses among women aged 50 years or older.
Among women younger than 50 years, the odds ratio for breast cancer in those with a history of induced abortion compared with those with no induced abortion history was 1.00. Among women aged 50 years or older, the corresponding odds ratio was 0.95. Miscarriage history similarly showed no association with breast cancer risk in either age group.
Researchers also examined whether breast cancer risk varied by the number of induced abortions or miscarriages, or by the age at which the first induced abortion or miscarriage occurred. They found no statistically significant variation in risk based on these factors.
One subgroup finding suggested a slightly increased risk of postmenopausal breast cancer among women whose first induced abortion occurred before age 20 years. However, researchers cautioned this may represent a chance finding, given the 30-year interval between the abortion and breast cancer diagnosis in this subgroup.
The study also reproduced expected associations for other established risk factors, which the researchers cited as supporting the reliability of the dataset and analytic approach. Earlier age at first birth showed a protective association on premenopausal breast cancer risk, and postmenopausal hormone therapy use – particularly long-term estrogen plus continuous progestin therapy – was associated with higher breast cancer risk.
The researchers emphasized that the topic remains a source of ongoing misinformation, including in some US state counseling materials that continue to suggest a link between abortion and breast cancer risk. They noted that the current findings align with prior large-scale analyses and prospective studies that have not supported the hypothesis.
The researchers concluded that induced abortion and miscarriage are not risk factors for subsequent breast cancer and that the findings may help provide reassurance in clinical counseling settings.
The authors declared having no competing interests.