The link between abortion and breast cancer risk is controversial. While several studies Abortion and Breast Cancer | Breast Cancer Resource Center | Imaginis - The Women's Health & Wellness Resource Network

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Abortion and Breast Cancer

The link between abortion and breast cancer risk is controversial. While several studies over the past few decades have explored the possible connection between abortion and breast cancer risk, the results have been contradictory. Some studies have shown a small increase in the risk of breast cancer in women with a history of abortions while other studies have found no such risk. Even some studies that have shown that a history of abortions can increase a woman's risk for breast cancer have been criticized because factors in these studies (such as reporting bias) may have contributed to inaccurate results.

According to independent experts at the National Cancer Institute and the American Cancer Society, there is currently no conclusive link between induced abortions and breast cancer risk. Despite these statements, abortion and breast cancer risk continues to be a controversial subject.

Why Has Abortion Been Linked To Breast Cancer Risk?

Breast cancer is one of the most common cancers among women and the second leading cause of cancer death in the United States (after lung cancer), excluding non-melanoma skin cancers. Since 1973, when abortion became legal in the United States, the incidence of breast cancer in the U.S. has risen dramatically.

While there has not been any conclusive evidence that a history of abortion increases a woman's risk of breast cancer, abortions have been suggested by some to increase breast cancer risk because of the involvement of the female hormone estrogen.

Near the beginning of pregnancy, estrogen levels increase so that the breasts may prepare to produce milk. After the woman gives birth, estrogen levels decrease again. In fact, full-term pregnancy (especially at a young age) has been associated with a decreased risk of breast cancer in medical studies. However, there has been some concern that if the pregnancy is aborted, a woman's estrogen levels would continue to remain elevated for a period of time, possibly increasing the risk of breast cancer. There is also some speculation (although not proven in human studies) that the breast duct cells, which normally go through a complete cycle of growth, development, and regression during pregnancy and lactation (breast-feeding), may be changed in some way, possibly leading to an increased risk of cancer.

There are two types of abortion: spontaneous (also called miscarriages) and induced (purposely performed). Spontaneous abortions that occur within the first three months of pregnancy are often due to insufficient estrogen levels. However, some believe that unlike spontaneous abortions, induced abortions may increase breast cancer risk because estrogen levels have been elevated and may remain elevated. While abortion and breast cancer risk is controversial, the link has not been conclusively proven.

In fact, a large epidemiological study published in the New England Journal of Medicine in 1997 showed that the risk of developing breast cancer for women with a history of induced abortion did not differ from the risk for women without a history of induced abortion. In this study, the researchers were able to avoid recall bias (women may not accurately report their reproductive history) since the information on abortions was collected before breast cancer developed.

Aside from the issue of abortions, scientists have identified several other risk factors for breast cancer, including:

  • Age
  • Family and personal history
  • Genetics
  • Having the first child after age 30 or never having children
  • Early menstruation (before age 12)
  • Late menopause (after age 50)
  • Smoking/alcohol use
  • Diet/obesity
  • Use of hormone replacement therapy

Click here for detailed explanations of the known risk factors for breast cancer. 

Why Are Abortion Studies Complicated?

Abortion studies are complicated because the issue of abortion is complex, influenced by emotional and socio-political components. Also, determining which abortion studies are valid and which are flawed can be difficult. Study results can be influenced by a number of factors, including:

  • The number of cases observed; a very small number may not be reflective of the general population)
  • The type of abortion: induced versus spontaneous (also called miscarriages)
  • Accounting for other lifestyle or genetic factors that may influence breast cancer risk
  • The amount of time between abortion and breast cancer occurrence
  • Data limitations due to the changing legal status of abortion (for example, an American woman’s abortion history may not have been reported before 1973, when the Supreme Court legalized abortion).
  • Recall bias (women may not accurately report their reproductive history)
  • Emotional and/or socio-political factors

Most early studies on abortion and breast cancer risk relied on women’s reports of their medical and reproductive history. However, the results of the studies might be inaccurate if the women inaccurately reported their history of abortions (called recall bias).

According to the National Cancer Institute, women with breast cancer may be more likely to accurately report sensitive reproduction issues such as abortions than women without breast cancer. This could make the results of some studies appear as though women who have breast cancer more commonly have a history of abortion, which may not be the case.

For example, in a study published in the Journal of the National Cancer Institute in 1996, researchers found a 90% increase in breast cancer risk (1.9 times the average risk, also known as relative risk) after induced abortions. However, the researchers suggested that this figure could be influenced by inaccurate recall and underreporting of abortions by the women in the study who did not have breast cancer since the study was conducted in a religiously conservative region of The Netherlands.

Another study published in the Journal of the National Cancer Institute in 1994 also shows the potential inaccuracies of abortion studies. In the study, researchers relied on self-reports of abortions from the women. The results showed a 50% increased risk of breast cancer (1.5 times the relative risk) from induced abortions. However, the study did not take into account a number of other potentially important factors, including the number of abortions the women had, the women's age at the time of the abortions, and the length of pregnancy before the abortions. Furthermore, the study did not show any increased risk between spontaneous abortions (miscarriages) and breast cancer risk. Because of the possible inaccuracies, the researchers said that the study did not permit scientific conclusions.

Note: A woman’s relative risk for breast cancer can increase due to a number of factors. For example, women with a first-degree relative with breast cancer (sister or mother) have the relative risk of 3.0 to 5.0 for developing breast cancer compared to women who do not have a first degree relative with breast cancer. Therefore, a relative risk of 1.9 or 1.5 for women with a history of induced abortions is small when compared to other breast cancer risk factors.

Recent Research

In October 2006, researchers published the results of a study of 267,361 women recruited into the European Prospective Investigation into Cancer (EPIC) and nutrition between 1992 and 2000. The data were collected from 20 centers in 9 countries. The results showed that history of abortion did not significantly increase breast cancer risk.

The National Cancer Institute will continue to support research investigating the relationship between hormones, particularly hormonal changes during pregnancy, and breast cancer.

Additional Resources and References

Update: August 2010