Abortion and breast cancer are both topics that can bring out strong
emotions in people. The issue of abortion generates passionate personal
and political viewpoints, regardless of a possible disease connection.
Breast cancer is the most common cancer in women. It can be a
life-threatening disease--one that that many women fear.
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
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
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
There are different types of abortion:
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.
is usually considered to be the death of a fetus after 5 months'
gestation while still in the uterus (womb).
abortion is probably what most people consider "abortion."
This refers to a woman's choice to end a pregnancy through a medical
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
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
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
(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
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
- Recognized spontaneous abortion is not associated with an
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."
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.
ACOG Committee on Gynecologic Practice. ACOG Committee
Opinion. Number 285, August 2003: Induced abortion and breast cancer
risk. Obstet Gynecol.
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.
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.
National Cancer Institute. Summary Report: Early Reproductive
Events and Breast Cancer Workshop. Available at:
Accessed May 8, 2007.