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The claim that undergoing an abortion increases a woman's chances of contracting breast cancer, often cited by anti-abortion activists, arguably has two origins: a scientific one and a political one. Its scientific origins have their roots in the 1980s and 1990s, with a series of studies investigating a link between induced abortions and an increased risk of breast cancer. Those early studies were ambiguous — no matter what side of the abortion issue one was on, one could (and still can) find a study to support their preferred narrative. From a scientific standpoint, the issue is no longer ambiguous. After an authoritative 1997 study utilizing government-collected data on every Danish women born between 1935 and 1978 concluded there was no increased risk of breast cancer from abortions, organizations such as the World Health Organization, U.S. National Cancer Institute, American Cancer Society, and many others now reject the existence of any such link. The conclusions drawn from that study, as published in the New England Journal of Medicine (NEJM), were that: From a political standpoint, the issue saw a resurgence in attention under the socially conservative administration of George W. Bush, which changed the language on an National Cancer Institute fact sheet regarding breast cancer and abortions against the wishes of that organization's scientists. This issue was discussed, later, in an August 2003 US Congressional report from the Committee on Oversight and Government Reform that investigated changes to science policy under the Bush administration: The notion that reproductive choices affect breast cancer risk is not new, nor is it controversial. It is widely accepted, for example, that carrying a pregnancy to term reduces one's risk of developing breast cancer later in life. This issue is often conflated with the question of abortions increasing a woman’s risk, because since carrying a pregnancy to term may reduce risk of breast cancer, having an abortion may offset that reduced risk. That is not the question at hand, however: the question, strictly defined, is ‘will having an abortion make you more likely to get breast cancer when compared to women who never give birth?’ The scientific basis for the breast cancer-abortion link has its roots, most notably, in a 1980 paper in the American Journal of Pathology that investigated one potential mechanism proposed for relationships between abortion and breast cancer: that leftover undifferentiated cells in a woman’s breast (which occur as part of hormonal changes that help to prepare a woman for breastfeeding and appear during pregnancy, and which would become fully differentiated by the end of a pregnancy) are more susceptible to cancer. To test this notion, researchers intentionally injected laboratory rats with a carcinogen after interrupted pregnancies, and then compared these results to a series of rats who were never pregnant, and to rats that carried pregnancies to full term, to see if interrupted pregnancies were related to an increased susceptibility to developing cancerous cells. The researchers demonstrated that the likelihood of developing cancerous cells was higher in those rats with interrupted pregnancies. The researchers suggested these results were consistent with the idea that the interruption of a pregnancy leaves undifferentiated cells in a woman’s breast, and that these cells are more susceptible to cancer: Since then, a goal of many researchers has been to design studies to see if this mechanism is valid and if it may be relevant in humans as well. For obvious reasons, a carcinogen injection kind of experiment on humans is out of the question, so scientists have to settle for observational studies. The different ways in which one can go about designing such studies have created a cornucopia of results with differing levels of veracity that anyone can point to, in isolation, to support their narrative. In terms of research on humans, there are two kinds of observational studies that would be used: case-control studies and cohort studies. In general, case control studies are smaller, and the way in which their control group is identified has the potential to be more subjective or error prone. The National Cancer Institute defines both as follows: Dominantly, studies suggesting a link fall into the case-control study design, which are problematic, according to an extensive 2004 review in The Lancet: An illustration of the quantitative effects such misreporting — sometimes referred to as a recall bias or reporting bias — could have on final results can be found in a 1996 study whose target population, by design, included both a conservative Catholic region and more socially liberal areas. Though this small, case-control study did show a correlation between abortions and breast cancer risk when comparing populations of women who have never given birth, the study’s more significant contribution was demonstrating the effect of recall bias on this kind of study: The 1997 NEJM study was a cohort study and is generally cited as the authoritative study on the alleged risk of abortion. It avoided reporting bias entirely by not conducting interviews at all, instead relying only on Danish data collected by the Danish government: That doesn’t mean you won’t find studies or individuals still suggesting a link between abortion and breast cancer. The non-profit Breast Cancer Prevention Institute (BCPI), founded by Joel Brind (one of the main scientists promoting the abortion and breast cancer connection), an organization whose sole purpose appears to be educating people about said connection, presents a list of studies that they say support their argument. This list is heavily referenced on anti-abortion websites and can commonly be found shared with the claim that 73 Studies Have Examined Abortion and Breast Cancer, 53 Show Higher Risk or similar. BCPI's is an extremely misleading representation of that information, however. Included among those (presently) 60 studies showing a higher risk of breast cancer from induced abortions, 23 were self-described on that organization's list as being not statistically significant. Suggesting that a statistically insignificant correlation supports any argument is, in a word, deceptive. That leaves 37 studies with statistically significant positive correlations: a ragtag collection of studies as old as 1957 and as recent as 2013, all of which were case-control or meta-analyses that relied primarily on case-controlled studies, and many of which were designed to test questions other than the abortion-breast cancer connection itself. One of the listed studies, discussed above, aimed to demonstrate (successfully) that case-control studies are a poor design to analyze a purported connection between abortions and breast cancer. Two of the listed studies included on the list were not, strictly speaking, studies, as they were conference presentations that received little to no peer review (i.e. Laing et al 1994, Bu et al 1995). Many of the studies on the list (e.g., Segi et al 1957, Daling et al 1994, Daling et al 1996) cautioned that while they may have demonstrated correlations, their data were not sufficient to establish causation or even to represent actual trends. Four of the studies on the list reached conclusions explicitly counter to the claim that they demonstrate an increased risk of breast in populations of women who have had abortions: Arguably the most notable publication on this list, both in scope and in citations, is a 1996 paper authored by Brind himself (who created this list in the first place), a meta-review of 28 studies (many of which are also featured individually on the list). His own study concluded: Typically, meta-reviews have stated inclusion standards delineating what studies are of high enough quality to be analyzed. In Brind’s study, however, no quality criteria were imposed, but a narrative review of all included studies is presented for the reader's use in assessing the quality of individual studies. Numerous scientists have serious issues with it on this basis. A team of Harvard epidemiologists used Brind’s methods in their own study, concluding that the causation he so confidently claimed the could not be made with the data he used. Mads Melbye, author of the 1997 Danish cohort study, took issue with Brind's study as well, as covered in a history of the topic: Our classification of false acknowledges that some scientists and studies suggest a link between abortion and breast cancer, but that suggestion is rooted in the fact that the methodologies utilized in the studies supporting such a link are widely accepted as flawed by the majority of the scientific community and by the fact that large cohort studies, better suited to test this question in the first place, suggest that no link exists. Observational studies of any kind will always come with limitations and wiggle room for politically motivated players to exploit, as was the case 2002. As a whole, however, we regard use of these data to make a causal link between breast cancer and abortions, without any discussion of their significant caveats involved, intentionally misleading enough to be disqualifying.
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