Is Abortion Linked to Breast Cancer?
Why is it hard to talk about abortion and breast cancer?
Abortion and breast cancer are both topics that can bring out strong feelings in people. The issue of abortion is often linked to personal and political viewpoints – even without a possible disease connection. Breast cancer is the most common cancer in women. It can be a deadly disease, and is one that many women fear.
Linking these 2 topics creates a great deal of emotion and debate. But scientific research studies have not found a cause-and-effect relationship between abortion and breast cancer.
How do hormones affect the breasts?
A woman's risk for some types of breast cancer is related to hormone levels in the body. Breast cells normally grow and divide in response to hormones like estrogen, progesterone, and prolactin. Levels of these hormones change throughout a woman's life, but can change a lot during pregnancy. When a woman is pregnant, her body gets ready for breast-feeding by altering the levels of these hormones. This causes changes in the breast tissue.
Does abortion affect hormones during pregnancy?
Concern about a possible link between abortion and breast cancer has been raised because abortion is thought to interrupt the normal cycle of hormones during pregnancy. Some believe that this interruption might increase a woman's risk of developing breast cancer.
There are different types of abortion:
- Spontaneous abortion, which most people refer to as a miscarriage, is the loss of a fetus before 5 months (20 weeks) into the pregnancy. It is often caused by problems with the fetus or with the maternal environment in which it is growing.
- Stillborn birth (stillbirth) is usually considered to be the death of a fetus after 5 months' gestation while still in the uterus (womb).
- Induced abortion is a medical procedure done to end a pregnancy. It is what most people think of when they hear "abortion."
All of these have been studied to see what effect they may have on a woman's risk of developing breast cancer later in life.
Research on abortion and breast cancer
Research study problems
Many studies have looked at a possible link between abortions and an increased risk of breast cancer. But because of the nature of the topic, these studies have been hard to do. This may help explain why some study findings vary.
Before 1973, induced abortions were illegal in much of the United States. So when researchers asked a woman about past pregnancies, she may not have felt safe saying that she had an illegal abortion. Even though abortion is now legal, it is still a very personal, private matter that many women do not like to talk about.
Studies have also shown that healthy women are less likely to report that they have had induced abortions. In contrast, women with breast cancer are more likely to accurately report their reproductive histories. This may be because they are searching their memories for anything that may be linked to the cancer.
The likelihood that women who have breast cancer will give a more complete account of their abortions than women who do not have breast cancer is an example of recall bias. Recall bias like this can sometimes cause studies to find links that may not exist.
Research study design
Most early studies of abortion and breast cancer used a case-control study design. This study design is very prone to recall bias (see above). In these studies, women with and without breast cancer were asked about past abortions. The researchers then compared the frequency of abortions in women with breast cancer (the cases) to those in women without breast cancer (the controls). It is likely that the higher rates of reported abortions in breast cancer cases (versus the controls), as seen in some of these studies, were not true findings because of recall bias. Studies that ask women about abortion before breast cancer is diagnosed do not seem to find this link.
A prospective study design is stronger and less prone to bias. In this type of study, a group of women who are cancer-free are asked about their past abortions and then are watched over a period of time to see if a new cancer occurs. In this type of study all of the women are cancer-free at the start, so there is no chance that having cancer will influence their memory of past abortions or their willingness to report past abortions.
Some prospective studies have further addressed the problem of recall bias by using new ways to find out about past induced abortions. For example, one study used birth certificates of children born to women with breast cancer to identify women who had had induced abortions. (The number of previous abortions was listed on these birth certificates.) This study found no increase in breast cancer risk in women who had an abortion followed by a live birth.
Research has confirmed that study type is likely to play a role in what is found. A review of the previous studies on this issue (see the next paragraph), covering tens of thousands of women, showed that women followed in prospective studies (which are less prone to recall bias) had no increased breast cancer risk if they had had an abortion. Case-control studies, which are prone to recall bias, pointed to a slight increase in risk.
Results from major prospective studies
The largest, and probably the most reliable study on this topic was done during the 1990s in Denmark, a country with very detailed medical records on all its citizens. In that study, all Danish women born between 1935 and 1978 (a total of 1.5 million women) were linked with the National Registry of Induced Abortions and with the Danish Cancer Registry. All of the information about their abortions and their breast cancer came from registries – it was very complete and was not influenced by recall bias.
After adjusting for known breast cancer risk factors, the researchers found that induced abortion(s) had no overall effect on the risk of breast cancer. The size of this study and the manner in which it was done provides good evidence that induced abortion does not affect a woman's risk of developing breast cancer.
Another large, prospective study was reported on by Harvard researchers in 2007. This study included more than 100,000 women who were between the ages of 29 and 46 at the start of the study in 1993. These women were followed until 2003. Again, because they were asked about childbirths and abortions at the start of the study, recall bias was unlikely to be a problem. After adjusting for known breast cancer risk factors, the researchers found no link between either spontaneous or induced abortions and breast cancer.
The California Teachers Study also reported on more than 100,000 women in 2008. Researchers asked the women in 1995 about past induced and spontaneous abortions. While the women were being followed in the study, more than 3,300 developed invasive breast cancer. 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.
What do the experts say?
In February 2003, the US National Cancer Institute (NCI) held a workshop of more than 100 of the world's leading experts who study pregnancy and breast cancer risk. The experts reviewed human and animal studies that looked at the link between pregnancy and breast cancer risk, including studies of induced and spontaneous abortions. Some of their findings were:
- Breast cancer risk is increased for a short time after a full-term pregnancy (that is, a pregnancy that results in the birth of a living child).
- Induced abortion is not linked to an increase in breast cancer risk.
- Spontaneous abortion is not linked to an increase in breast cancer risk.
The level of scientific evidence for these findings was considered to be "well established" (the highest level).
The American College of Obstetricians and Gynecologists (ACOG) Committee on Gynecologic Practice also reviewed the available evidence in 2003 and again in 2009. ACOG published its most recent findings in June 2009. At that time, the Committee said, "Early studies of the relationship between prior induced abortion and breast cancer risk were methodologically flawed. More rigorous recent studies demonstrate no causal relationship between induced abortion and a subsequent increase in breast cancer risk."
In 2004, the Collaborative Group on Hormonal Factors in Breast Cancer, based out of Oxford University in England, put together the results from 53 separate studies done in 16 different countries. These studies included about 83,000 women with breast cancer. After combining and reviewing the results from these studies, the researchers concluded that "the totality of worldwide epidemiological evidence indicates that pregnancies ending as either spontaneous or induced abortions do not have adverse effects on women's subsequent risk of developing breast cancer." These experts did not find that abortions (either induced or spontaneous) cause a higher breast cancer risk.
The topic of abortion and breast cancer highlights many of the most challenging aspects of studies of people and how those studies do or do not translate into public health guidelines. The issue of abortion generates passionate viewpoints in many people. Breast cancer is the most common cancer, and is the second leading cancer killer in women. Still, the public is not well-served by false alarms. At this time, the scientific evidence does not support the notion that abortion of any kind raises the risk of breast cancer.
American College of Obstetricians and Gynecologists (ACOG) Committee on Gynecologic Practice. ACOG Committee Opinion. No. 434: Induced Abortion and Breast Cancer Risk. Obstet Gynecol. 2009;113:1417-1418.
Beral V, Bull D, Doll R, et al. Collaborative Group on Hormonal Factors in Breast Cancer. Breast cancer and abortion: Collaborative reanalysis of data from 53 epidemiological studies, including 83,000 women with breast cancer from 16 countries. Lancet. 2004;363:1007-1016.
Henderson KD, Sullivan-Halley J, Reynolds P, et al. Incomplete pregnancy is not associated with breast cancer risk: The California Teachers Study. Contraception. 2008;77:391-396.
Melbye M, Wohlfahrt J, Olsen JH, et al. Induced abortion and the risk of breast cancer. N Engl J Med. 1997;336:81-85.
Michels KB, Xue F, Colditz GA, Willett WC. Induced and spontaneous abortion and incidence of breast cancer among young women. Arch Intern Med. 2007;167:814-820.
National Cancer Institute. Abortion, Miscarriage, and Breast Cancer Risk. Accessed at www.cancer.gov/cancertopics/factsheet/Risk/abortion-miscarriage on September 20, 2010.
National Cancer Institute. Summary Report: Early Reproductive Events and Breast Cancer Workshop. Accessed at www.cancer.gov/cancerinfo/ere-workshop-report on September 20, 2010.