Updated June 2011
It has been hypothesized that surgical (induced) and spontaneous abortions (miscarriages) increase breast cancer risk. However, the largest and most reliable research studies show that there is no association between either kind of abortion and risk of breast cancer. Based on overwhelming scientific evidence, NBCC does not support any public policy efforts that imply such a link exists.
Dozens of studies have examined the relationship between abortion and breast cancer. Though some studies have suggested that abortion increases the risk of breast cancer, many of these were severely flawed and their results are not valid. In these studies, the researchers asked women whether they had had abortions and did not verify the self-reports with medical records. Due to social and political sensitivity about abortion, women’s reporting of their own abortion history may be unreliable. On the other hand, women seeking explanations about the source of their breast cancer may be highly motivated to disclose as much information about their health history as they can. This phenomenon is called reporting bias, and it could lead to incorrect conclusions about a relationship between abortion and breast cancer. At least two studies of abortion and breast cancer risk have provided direct evidence of the existence of exactly this type of reporting bias.
In 1997 researchers in Denmark published the results of a large, well-done cohort study of abortion and breast cancer risk. This study, which included data on 1.5 million women, avoided the problem of reporting bias by relying on data collected from abortion registries (i.e. medical records) rather than individual reports. This study found no association between induced abortion and breast cancer risk; women who had abortions were no more likely to develop breast cancer than women who had not had abortions. This finding was consistent among various subgroups of women; it did not differ depending on age at abortion, number of children, time since abortion, or age at diagnosis of breast cancer.
Several other studies have also found no association between abortion and breast cancer risk, including two studies conducted in China. Though these studies relied on self-reports to assess which women had abortions, reporting bias was most likely minimal. Abortion is legal, common and socially acceptable in China, so women do not feel stigmatized and may be more likely to give an accurate answer as to whether or not they have had an abortion.
More recently, researchers followed more than 100,000 women in the Nurses’ Health Study II cohort from 1993 to 2003 for breast cancer occurrence.6 Women were asked about childbirths and abortions at the start of the study; therefore eliminating recall bias. Consistent with previous studies, these researchers found no association between either spontaneous or induced abortions and breast cancer in this predominantly premenopausal population of women.
Finally, in 2008, researchers further informed the debate by examining invasive breast cancer as it relates to induced abortions and miscarriages among 109,893 women participating in the California Teachers Study (CTS) cohort. At baseline, researchers asked the women about past induced and spontaneous abortions. While participants were followed from 1995 through 2004, over 3,300 women developed invasive breast cancer. Once again, there was no difference in breast cancer risk between the group who had either spontaneous or induced abortions and those who had not had an abortion.
In February 2003, the National Cancer Institute (NCI) convened a workshop to review the research and to make conclusions about the relationship between reproductive factors and breast cancer risk. Over 100 world experts from both the scientific research community and the breast cancer advocacy community, including several NBCC Board members, were invited to participate in the workshop.
After reviewing the evidence, the workshop attendees issued a report stating that there is strong evidence that neither spontaneous nor induced abortion increases the risk of breast cancer. The report’s findings were reviewed and unanimously approved by NCI’s Board of Scientific Advisors and Board of Scientific Counselors.
The American College of Obstetricians and Gynecologists (ACOG) Committee on Gynecologic Practice also reviewed the available evidence on induced abortion and breast cancer risk in 20032 and again in 2009. In the most recent review, the Committee came to the conclusion that “more rigorous recent studies demonstrate no causal relationship between induced abortion and a subsequent increase in breast cancer risk.”
NBCC believes that breast cancer policies should be based on quality scientific evidence, and that the assessment of this evidence should not be influenced by political ideology. The highest quality studies show that there is no link between abortion and breast cancer risk. Thus, public policy efforts should be aimed at educating women that abortion will not increase their risk of breast cancer.