British researchers reviewed studies involving 83,000 women and found that pregnancies ending in abortion or miscarriage (also known as spontaneous abortion) do not significantly affect a woman's risk of developing breast cancer.
"This is the first time that the vast majority -- over 90% of studies that have ever looked at the relationship between abortion and breast cancer -- have been brought together and looked at," says researcher Valerie Beral, professor of epidemiology at the University of Oxford, England. "Overall, we found quite clear evidence that there's no increased risk of breast cancer from either miscarriage or abortion."
Researchers say previous studies that suggested having an abortion slightly increases a woman's risk of breast cancer were flawed because they were based on women reporting having had an abortion after a diagnosis of breast cancer, known as a retrospective study.
"Studies can give misleading results if women are asked about previous abortions only after they are diagnosed with breast cancer," says researcher Richard Peto of Collaborative Group on Hormonal Factors in Breast Cancer, in a news release. "This may well be because, on average, women with breast cancer are more likely than other women to disclose any prior induced abortions."
Therefore, their analysis relied primarily on studies based on information on abortion collected before the diagnosis of breast cancer, known as a prospective data.
Despite the results of this study, which agree with several recent consensus statements from major medical organizations, the controversy surrounding the issue isn't likely to falter due to the highly political nature of the abortion debate.
Sorting Through the Studies on Breast Cancer and Abortion
In the study, published in the March 27 issue of The Lancet, researchers analyzed data on individual women from 53 studies in 16 countries. Researchers compared the effects of having an abortion or miscarriage with those of never having been pregnant on breast cancer risk.
The study was funded by Cancer Research UK.
According to data from 44,000 women who participated in the studies analyzed, researchers found no increase in breast cancer risk among women who had a pregnancy that ended in miscarriage or abortion. In fact, women who had a pregnancy that ended in abortion had a slightly lower (7%) risk of breast cancer, and the number of abortions was not associated with any change in breast cancer risk.
"You could argue that if anything there is a small decreased risk, but I think the main thing is that it confirms very strongly that we can say quite confidently that there is no increased risk," says Beral.
Beral says the data collected from the 39,000 women who participated in retrospective studies were less reliable. Although the links between breast cancer and miscarriage were similar among the retrospective and prospective studies, the results on abortion and breast cancer varied widely between the two types of studies.
"If you've got something we know is correct, and something we're not sure about," says Bernal referring to the studies that look at data collection prospectively and retrospectively, "and we get a one-in-a-million chance that these results are similar, it suggests that the other ones are biased."
Researchers have been studying the relationship between abortion and breast cancer for nearly 50 years. Until the mid-1990s, studies on the issue had produced inconsistent results, and most of the studies were considered flawed because they involved a small number of women and many studies collected information only after breast cancer had been diagnosed.
In 1996, Joel Brind, PhD, a professor of biology and endocrinology at Baruch College of the City University of New York, published a study based on 23 independent studies that asked women with and without breast cancer whether they had ever had an abortion.
Brind's study suggested that having had an abortion increased a woman's risk of breast cancer.
But Brind takes issue with the Beral's study and says the studies used in the analysis are also flawed.
Brind tells WebMD that studies that collect data prospectively tend to have huge gaps of information within the data. He says this leads to flawed results and conclusions.
"The prevalence of abortion is underestimated such that many, many women ... who had an induced abortion who are in that study are in fact known and proved to be misclassified as not having had an induced abortion."
But Brind says when those studies appear in internationally recognized medical journals, "people take them as gospel."
"In a non-politically sensitive field, such junk science would never make it into high-quality ... journals like that," says Brind.
Since Brind's study was published, a variety of medical and government organizations have conducted extensive reviews of the available research on the relationship between breast cancer and abortion and excluded any cause-and-effect relationship between the two.
- February 2003: An international scientific panel convened by the National Cancer Institute concluded that having an abortion or miscarriage "does not increase a woman's subsequent risk of developing breast cancer."
- August 2003: The American College of Obstetricians and Gynecologists concluded that although studies on the issue were inconsistent and difficult to interpret, mainly due to study design flaws, "there is no evidence supporting a causal link between induced abortion and subsequent development of breast cancer."
- September 2003: The American Cancer Society updated the cancer reference information on its web site to include the above findings and concluded, "At the present time, the scientific evidence does not support a causal association between induced abortion and breast cancer."
With the overwhelming agreement of major health organizations and inherent complexity of conducting research on this issue, Beral says her study should come as close as researchers can possibly get to providing a conclusive answer to the big question.
"This rules out short-term pregnancies ending in abortion as having a major role in breast cancer," Beral tells WebMD.