The alleged association between induced abortion and risk of breast cancer: biology or bias?

The alleged association between induced abortion and breast cancer is one of the most controversial and important questions in women's health today. To help clinicians provide appropriate counseling, we reviewed both the primary data and review articles on this topic. We identified the studies and reviews by using Medline and the reference lists of articles and texts. We then used the U.S. Preventive Services Task Force rating system to evaluate the evidence. Many case-control studies have addressed this question, but their results have been inconsistent. Persistent problems in the case-control studies include selection of an appropriate control group, recall bias (under-reporting of induced abortion by controls), and confounding by other risk factors. Two recent, large cohort studies, which are less susceptible to bias, showed either protection or no effect on breast cancer risk from an induced abortion. At present, level II-2 evidence (cohort and case-control studies) supports a class B recommendation (fair evidence) that induced abortion does not increase a woman's risk of breast cancer later in life.