Linking these 2 topics understandably creates a great deal of emotion, as well as controversy. Research studies, however, have not found a cause-and-effect relationship between abortion and breast cancer.

Background

A woman's risk of developing breast cancer is related to hormone levels in the body. Breast cells normally grow and divide in response to the levels of certain hormones, such as estrogen, progesterone, and prolactin. Levels of these hormones change throughout a woman's life, but can change a great deal during pregnancy. When a woman is pregnant, her body begins to prepare for breast-feeding by altering the levels of these hormones. This causes changes in the breast tissue.

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. This interruption is believed by some to 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') gestation. 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 probably what most people consider "abortion." This refers to a woman's choice to end a pregnancy through a medical procedure.

All of these situations 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 difficult to conduct. This may help explain why some have reached different conclusions.

Before 1973, induced abortions were illegal in much of the United States. Therefore, when researchers asked about a woman's reproductive past, women may not have been comfortable disclosing the fact that they had an illegal abortion. Even though abortion is now legal, it is still a very personal, private matter that many women are hesitant to talk about.

Studies have shown that healthy women are less likely to report their histories of induced abortions. In contrast, women with breast cancer are more likely to accurately report their reproductive histories because they are searching their memories for anything that may have contributed to their disease.

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 called "recall bias," and it can seriously undermine the accuracy of study results.

Research Study Design

Most early studies of abortion and breast cancer used a case-control study design, one that is very prone to recall bias. In these studies, women with and without breast cancer were asked to report 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 (vs. controls) observed in many of these studies were not true findings because of recall bias.

A prospective (cohort) 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 observed 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 the disease will influence their memory of past abortions or willingness to report past abortions.

Some prospective studies have addressed the problem of recall bias by using innovative ways to document induced abortions. For example, a recent 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 whose abortion is followed by a live birth.

Recent research has confirmed that the type of study likely plays a role in what is found. A review of the previous studies on this issue, covering tens of thousands of women, showed that women followed in prospective studies (which are less prone to bias) had no increased breast cancer risk if they had had an abortion. Retrospective (case-control) studies, on the other hand, pointed to a slight increase in risk.

Research Study Results

The largest, and probably the most reliable, single study of this topic was conducted 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 (1.5 million women) were linked with the National Registry of Induced Abortions and with the Danish Cancer Registry. So all information about their abortions and their breast cancer came from registries, 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 substantial 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 their reproductive history 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.

What 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 existing human and animal studies on the relationship between pregnancy and breast cancer risk, including studies of induced and spontaneous abortions. Among their conclusions were:

• Breast cancer risk is temporarily increased after a term pregnancy (that is, a pregnancy that results in the birth of a living child). 
• Induced abortion is not associated with an increase in breast cancer risk. 
• Recognized spontaneous abortion is not associated with an increase in breast cancer risk.

The level of scientific evidence for these conclusions was considered to be "well established" (the highest level).

The American College of Obstetricians and Gynecologists (ACOG) Committee on Gynecologic Practice reviewed the available evidence as well and published its findings in August 2003. The committee concluded that "early studies of the relationship between prior induced abortion and breast cancer risk have been inconsistent and are difficult to interpret because of methodologic considerations. More rigorous recent studies argue against a causal relationship between induced abortion and a subsequent increase in breast cancer risk."

The Collaborative Group on Hormonal Factors in Breast Cancer, based out of Oxford University in England, recently put together the results from 53 separate studies conducted 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."

Conclusion

The topic of abortion and breast cancer highlights many of the most challenging aspects of studies of human populations and how those studies do or do not translate into public health guidelines. The issue of abortion generates passionate viewpoints among 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 and at the present time, the scientific evidence does not support a causal association between induced abortion and breast cancer.

References

ACOG Committee on Gynecologic Practice. ACOG Committee Opinion. Number 285, August 2003: Induced abortion and breast cancer risk. Obstet Gynecol. 2003;102:433-435.

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.

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. Summary Report: Early Reproductive Events and Breast Cancer Workshop. Available at: www.cancer.gov/cancerinfo/ere-workshop-report. Accessed May 8, 2007.

Revised: 08/06/2007