Both abortion and breast cancer are 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, and it can be a life-threatening disease that most
women fear.
Linking these two topics understandably generates 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 fluctuate 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 it is thought to interrupt the normal cycle of hormones during
pregnancy. This interruption is believed by some to increase ones' 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," in
that a woman chooses to end a pregnancy.
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
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, which 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
literally 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.
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
frequency of abortions in women with breast cancer and the disease-free controls
was then compared. It is likely that the small increases in breast cancer risk
observed in many of these studies were not authentic findings because of recall
bias.
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 observed over a period of time to see if a new cancer
occurs. In this type of study, there is no chance that having the disease will
influence a woman’s memory of past abortions or willingness to report past
abortions.
Some prospective studies have solved 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 pregnancies and
their outcomes were listed on these birth certificates.) This study found no
increase in breast cancer risk in women whose abortion is followed by a live
birth.
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 conducted provides substantial
evidence that induced abortion does not affect a woman's risk of developing
breast cancer.
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 (see below), 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.
What the Experts Say
In February 2003, the US National Cancer Institute (NCI) convened a workshop
of over 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 transiently (temporarily) increased after a term
pregnancy [resulting 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 their findings in August 2003. They 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, even when both the exposure and the disease are of
great importance and interest to us all. 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(2):433-5.
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, Frisch M, Westergaard T, Helweg-Larsen K,
Andersen PK. Induced abortion and the risk of breast cancer. N Engl J Med. 1997;336(2):81-5.
National Cancer Institute. Summary Report: Early Reproductive Events and
Breast Cancer Workshop. Available online at: www.cancer.gov/cancerinfo/ere-workshop-report. Accessed September 2004.
Revised 10/06/04
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