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+ | ==========Studies that Deny the Abortion-Breast Cancer Link========== | ||
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+ | Often, those who deny the abortion-breast cancer link will cite the findings of a well-publicized study, such as the Melbye study (the Danish study), the Beral re-analysis, | ||
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+ | =====1. 1997 Melbye Study (the Danish Study)===== | ||
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+ | In January 1997, the Danish Melbye study was published in the prestigious //New England Journal of Medicine// | ||
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+ | The Melbye study is insufficient to answer the question of whether induced abortion has any adverse effect on women: It devotes a mere paragraph of text and one unsophisticated comparison to assess the effect of induced abortion (relative to no abortion history), it employs unsuitable comparisons to assess the influence of the number and timing of abortions procured, it possibly eliminates all effect of induced abortion by controlling for the time period at which abortions were procured, excludes women with in situ breast cancer, and fails to consider the pathology of breast cancer in assessing the timeframe in which the disease would manifest itself following an induced abortion. | ||
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+ | **__Unsophisticated analysis of induced abortion.__** The Melbye study states that “[o]verall, | ||
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+ | The authors reserve sophisticated modeling for a table in which they examine the marginal risks incurred by women based on the circumstances of their procured abortions. All women examined in this analysis have had at least one induced abortion; none of the women considered are without induced abortion history. Hence, this is not an analysis of the effects of induced abortion history relative to having no abortion history, but of the effects of the circumstances of an induced abortion relative to other circumstances. | ||
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+ | **__Unsuitable analyses.__** To address the effects of repeated induced abortions, Melbye et al. use women who have had only one abortion (76.8 percent of aborting women) as a reference group for women with two abortions (17.1 percent) or three or more abortions (6.1 percent). | ||
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+ | The authors’ neglect of women without abortion history also results in the lack of a suitable reference group for their analysis of the effects of the ordering of live births and abortions. Women who obtain abortions only after their first live birth are used as a reference group for aborting childless women, women who procure abortions only before their first live birth, and women who procure abortions both before and after their first live birth. | ||
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+ | The reference groups used in both the analysis of the effect of number of induced abortions and of timing of the induced abortions are unsuitable. Less than 19 percent of their sample had induced abortion history. The appropriate reference category to assess the effect of number and ordering of abortions is parous women with only full-term pregnancies. | ||
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+ | **__Reporting difficulty around abortion law change and control for abortion’s legality.__** Melbye and colleagues also applied a control that diminished the strength of their findings: Their results are controlled for the time period in which the induced abortion was procured. Abortion became legal in Denmark in 1939, | ||
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+ | **__Late induced abortion.__** As noted above, Melbye ((Mads Melbye, Jan Wohlfahrt, Jørgen H. Olsen, Morten Frisch, Tine Westergaard, | ||
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+ | **__Health or survivor bias.__** Melbye et al. risk introducing [[reporting_bias_in_abortion-breast_cancer_studies|health bias or survivor bias]] into their study by excluding women with in situ breast cancer; their study was restricted to women with invasive breast cancer. | ||
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+ | **__Incorrect time frames.__** Finally, the time frames established in the Melbye study for analysis of cancer development in aborting women are not tailored to the specific pathology of cancer growth. The authors analyzed the risk of cancer among women for whom under one year had passed since an induced abortion and for whom one to four years had passed. The cohort for whom five or more years had passed since an induced abortion was established as the reference category. Note that breast cancer resulting from an induced abortion would not be detectable until approximately eight to 10 years thereafter. Hence, induced abortion was not found to increase breast cancer risk under one or one to four years thereafter. Furthermore, | ||
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+ | =====2. 2001 Goldacre Study===== | ||
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+ | The UK Goldacre study is marked by incomplete reporting and distinguishing of spontaneous and induced abortions; omitted variable bias through the lack of empirical consideration of data on parity, age, and other breast cancer risk factors (and hence no parsing of the effects of differently-ordered abortions); an incompletely specified model; and insufficiently randomized data. Given these flaws, this study is not a significant contribution to the literature. | ||
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+ | **__Incomplete reporting and distinguishing of induced and spontaneous abortions.__** Furthermore, | ||
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+ | **__Incomplete model (lack of parity data).__** The authors have only incomplete data on abortions and also note that analysis of certain “lifestyle or reproductive variables [was] outside the scope of [their] study.” Among these variables is pregnancy. Though the authors assert that they closely matched control groups to cases for data on “these factors” (e.g., “reproductive and lifestyle variables”), | ||
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+ | **__Unsophisticated analysis of induced abortion.__** Furthermore, | ||
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+ | **__Reporting difficulty around abortion law change, sample age, and expected number of abortions.__** The women with tabulated breast cancer incidence under investigation include some cohorts without many abortions during their fertile years (older women, who were fertile before abortion was legal in the UK) and those with relatively many during their fertile years (younger women, fertile after abortion was legal). Even though age is stratified (analogous to controlling for age), the recombination of these cohorts’ incidence rates will interact age (one being older or younger) with abortion.((Here is a putative interaction. Women in their 30s in the 1960s are only in their 60s in the 1990s. All those older (later 60s, or in their 70s or even 80s) will have statistically unmeasurable abortion rates. (In the 1960s, when abortion was legalized, these older women were too late, relative their fertile period, to demand abortions. The diminished number of abortions in their population is evident: As noted below, whereas barely 1 percent of their cases had an abortion over 30 years, the abortion rate in the UK was over one percent per year across those 30 years!) These older women have a plurality of the breast cancer cases. Older women get more breast cancer. In the calculation of " | ||
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+ | Goldacre’s “expected” breast cancer incidence comes from the general population’s rate (though they do not explain how this number is derived). Their “observed” breast cancer rate is what they measure for those who have had an abortion. But the two quoted rates for the general population and aborting women have very different women in them: These statistics aggregate (recombine) women of very different age types. Those having had an abortion are much younger (because abortion was generally available and legal only after 1967). The aborting women, being younger, will exhibit (what is “observed” in their class) lower breast cancer rates. Younger women get breast cancer much less frequently than older women. | ||
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+ | Once (what appears to be) the window has passed for a breast cancer to manifest itself (once 14 years or so have passed), these women (older women) show a decreased risk of breast cancer. The model shows this, albeit imprecisely. | ||
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+ | **__Incomplete and insufficiently randomized data.__** Finally, the sample suffers from selection bias, as it was confined to women who obtained abortions in hospitals. As Joel Brind notes in a separate review, a mere 300 of the 28,616 cases included in Goldacre (women diagnosed with breast cancer between 1968 and 1998) were classified as having a history of induced abortion—amounting to barely 1 percent of cases over a 30-year period. However, the abortion rate in the UK was over 1 percent per year over that period; | ||
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+ | =====3. 2004 Beral Re-analysis===== | ||
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+ | Valerie Beral’s large “re-analysis” of data from 53 epidemiological studies, including 83,000 women with breast cancer from 16 countries, was published in the British journal //The Lancet// in 2004.((V. Beral, D. Bull, R. Doll, R. Peto, G. Reeves, 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, | ||
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+ | The Beral re-analysis is flawed by two instances of publication bias: The authors unsystematically dismissed the result of their analysis of retrospective data in favor of their analysis of prospective data, and they unsystematically excluded certain peer-reviewed studies from their analysis. Beral and colleagues also chose an unsuitable reference group to assess any influence of induced abortion on breast cancer, excluded studies including women with in situ breast cancer, and failed to distinguish between first- and second-trimester spontaneous abortions. | ||
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+ | **__Induced abortion.__** Beral and colleagues found induced abortion history contributed to a statistically significant decrease in breast cancer risk in their meta-regression of studies based on prospectively collected data and a statistically significant increase in breast cancer risk in their meta-regression of studies based on retrospectively collected data. | ||
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+ | No significant influence was found, in prospectively collected or in retrospectively collected data, for two or more induced abortions relative to one induced abortion, for experiencing a first induced abortion before age 25 relative to after age 25, for an induced abortion being fewer than 10 years in the past versus an induced abortion being 10 or more years in the past, or induced abortion before versus after giving birth. | ||
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+ | **__Spontaneous abortion.__** The Beral re-analysis found no significant effect for spontaneous abortion in either their analysis of studies based on prospective data or in their analysis of studies based on retrospective data. | ||
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+ | **__Publication bias.__** As explained above, the Beral study is marked by several flaws, including two types of publication bias. The first type is the dismissal of findings sourced in retrospective data; the second is the unsystematic exclusion of certain datasets from their meta-analysis. | ||
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+ | **__Publication bias: Dismissal of analysis of retrospective data.__** Beral and her colleagues divided the studies they analyzed into two separate categories: Those that used retrospective methods of data collection (i.e., information from patients after they were diagnosed with breast cancer and control subjects) and those that used prospective methods (i.e., medical records taken before a breast cancer diagnosis). As noted above, the 39 retrospective studies showed evidence of an increase in breast cancer risk with abortion. The 13 prospective studies showed a decreased breast cancer risk with abortion. The study concluded from its prospective data that there was no association between induced abortion and breast cancer, and this conclusion was widely reported in the press. Instead of reporting the results of their study accurately, the authors in their conclusion termed the increase in breast cancer risk based on retrospective data “misleading” and asserted that “recall bias” altered the data.((V. Beral, D. Bull, R. Doll, R. Peto, G. Reeves, 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, | ||
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+ | The authors’ unsystematic dismissal of their findings based on retrospective data is scientifically unjustifiable because it is arbitrary. Though it is possible that healthy women in retrospective studies underreport their abortions, it is also possible (and the authors admit as much) that underreporting of abortions took place in the prospective studies. They offer no substantiation for their statement that underreporting would not significantly distort prospective studies. Yet the authors do not dismiss the result of their analysis based on prospective data. Rather than dismissing the result of their analysis of retrospective data, they could have built controls for the circumstances under which the data in each study were obtained into their model and thereby controlled for recall bias. They did not. Their dismissal of findings sourced in retrospective data is based on an arbitrarily applied assumption. | ||
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+ | The authors state, “In view of the potential for differential retrospective reporting of past induced abortions to distort the results, and given the highly significant differences found here between the overall findings about the studies that had recorded information on induced abortion retrospectively and prospectively, | ||
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+ | **__Publication bias: Unscientific exclusion of studies.__** Beral and colleagues were also unsystematic in choosing which datasets to include and exclude. Beral et al. deliberately excluded a total of 13 peer-reviewed studies from their analysis. They also failed to note the existence at least five published datasets.((A.E. Laing, G.E. Bonney, L. Adams-Campbell, | ||
+ | \\ D.G. Zaridze data (unpublished) in N. Andrieu, S.W. Duffy, T.E. Rohan, M.G. Lê, E. Luporsi, M. Gerber, R. Renaud, D.G. Zaridze, Y. Lifanova, and N.E. Day, “Familial Risk, Abortion and Their Interactive Effect on the Risk of Breast Cancer—A Combined Analysis of Six Case-Control Studies,” //British Journal of Cancer// 72, no. 3 (1995): 744-751; | ||
+ | \\ E. Luporsi, “Breast Cancer and Alcohol,” (PhD thesis, University of Paris-Sud, 1988), data in N. Andrieu, S.W. Duffy, T.E. Rohan, M.G. Lê, E. Luporsi, M. Gerber, R. Renaud, D.G. Zaridze, Y. Lifanova, and N.E. Day, “Familial Risk, Abortion and Their Interactive Effect on the Risk of Breast Cancer—A Combined Analysis of Six Case-Control Studies,” //British Journal of Cancer// 72, no. 3 (1995): 744-751; | ||
+ | \\ L. Bu, L.F. Voigt, Z. Yu, K.E. Malone, and J.R. Daling, “Risk of Breast Cancer Associated with Induced Abortion in a Population at Low Risk of Breast Cancer,” //American Journal of Epidemiology// | ||
+ | \\ Lynn Rosenberg, Julie R. Palmer, David W. Kaufman, Brian L. Strom, David Schottenfeld, | ||
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+ | The authors included some unpublished studies and some unpublished abortion data in their analysis: “Only about two-thirds of the eligible studies that had obtained relevant information had published their findings on abortion and breast cancer.” Beral and her colleagues take the perspective that by including unpublished data they have avoided the risks associated with (a particular type of) publication bias((The type of publication bias they hope to avoid is the bias in journals against publishing studies with null results (i.e., those that show no effect for a given variable). This bias is a problem in epidemiology, | ||
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+ | For example, among the unpublished datasets Beral referenced is a Scottish study showing a decrease in breast cancer risk with abortion. As the study’s data were unpublished, | ||
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+ | **__Health or survivor bias.__** Notably, the authors included only studies of women with invasive breast cancer and excluded in situ breast cancer, the significance of this is addressed above. | ||
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+ | **__Unsuitable comparison.__** Another major flaw in the Beral study lay in its choice of reference group. The authors compare the risk of a pregnancy ending in induced abortion with the risk of “never having had that pregnancy.” Their language is unclear, but if Beral et al. here refer to nulliparity, | ||
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+ | Note also that inappropriate comparisons were set up for Beral et al.’s more sophisticated analyses. For these analyses, when testing the influence of number of induced abortions, age at first induced abortion, number of years since an induced abortion, or the ordering of induced abortions and live births, the preferred reference group is parous women with no induced abortion history. | ||
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+ | **__No distinction between first- and second-trimester spontaneous abortions.__** Finally, Beral et al. fail to distinguish, | ||
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+ | Given these serious methodological flaws and the confusion this study has caused, the best that can be done is to disregard this piece of research. It did not contribute to the steady march of scholarship or to clarity in epidemiological patterns of breast cancer development. Additionally, | ||
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+ | =====4. 2005 Brewster Study===== | ||
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+ | One prospective study used in the much-quoted Beral re-analysis study is the Brewster Scottish prospective study (of which Beral herself was a co-author).((David H. Brewster, Diane L. Stockton, Richard Dobbie, Diana Bull, and Valerie Beral, “Risk of Breast Cancer after Miscarriage or Induced Abortion: A Scottish Record Linkage Case-Control Study,” //Journal of Epidemiology and Community Health// 59, (2005): 283-287.)) The Brewster study is negatively affected by a glaring lack of data on parity, which diminishes its ability to distinguish the effect of differently timed induced abortions. The Brewster study introduced health bias into its analyses by including only “new incident breast cancers” and excluding women with a previous history of cancer, as well as excluding controls with cancer and women with a history of in situ breast cancer. | ||
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+ | **__Lack of data on parity.__** This study included women “with all reproductive events occurring from 1981 onwards[, and] … with some reproductive events occurring before 1981, and number of pregnancies equalled number of births—// | ||
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+ | **__Unsuitable comparisons.__** Furthermore, | ||
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+ | This comparison employs almost 10,000 women with no induced abortion history, over 1,700 women with an unclear sequence of induced abortions and pregnancies, | ||
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+ | Additionally, | ||
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+ | **__Health or survivor bias.__** Finally, Brewster et al. excluded women with any history of cancer or of in situ breast cancer prior to their “diagnosis of breast cancer/ | ||
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+ | =====5. 2007 Michels Study (the Harvard Nurses' | ||
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+ | The Michels study concluded that there was no increased risk of breast cancer with induced abortion.((Karin B. Michels, Fei Xue, Graham A. Colditz, and Walter C. Willett, “Induced and Spontaneous Abortion and Incidence of Breast Cancer among Young Women,” //Archives of Internal Medicine// 167, no. 8 (2007): 814-820.)) Because of the prestigious name of the dataset (the Nurses’ Health Study II) and the Harvard University affiliations of some of the authors, the study’s conclusions have had massive impact on the induced abortion-breast cancer debate despite its flaws, which are such that the study’s conclusion could actually be reversed. | ||
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+ | The Michels study suffers from the introduction of massive error through answers supplied by the authors to questions left half-blank, from unsuitable comparisons and the lack of distinction between first- and second-trimester spontaneous abortions, from follow-up time after (some fraction of) induced abortions insufficient to detect cancer, from sampling bias due to the study’s focus on educated women, and from health bias or survivor bias from the exclusion of women with a history of previous cancer or of in situ breast cancer. These (and possibly other) flaws are serious enough for this study to be, unhappily, discounted. | ||
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+ | The Michels study, which is based on data from the longitudinal Nurses’ Health Study II, includes over 100,000 female nurses. These women were initially surveyed in 1989. Ninety-two percent of these were white. | ||
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+ | **__Induced abortion.__** The Michels study found no significant influence for induced abortion on breast cancer risk, whether assessed generally, by the number of induced abortions, by the age at first induced abortion, or temporally (that is, with respect to the timing of first birth). No effect was distinguished when women were divided by parity status and then re-assessed according to their general induced abortion history and number of induced abortions. | ||
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+ | When induced abortions were broken down (among nulliparous and parous women) by potential relationship with specific types of breast cancer, induced abortion among parous women was found to have a positive, significant influence on the risk of PR-(progesterone receptor negative) breast cancer. | ||
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+ | **__Spontaneous abortion.__** The Michels study found no significant influence on breast cancer risk for spontaneous abortion or number of spontaneous abortions. It did, however, find a significantly protective effect for spontaneous abortions taking place at or before age 19 (but no other age). | ||
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+ | No effect for spontaneous abortions was found when women were distinguished by parity status and their general spontaneous abortion history and number of spontaneous abortions were assessed. | ||
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+ | However, when assessed temporally, spontaneous abortions after first birth were found to have a marginally significantly protective (i.e., negative) influence on breast cancer risk. Spontaneous abortions before first birth were not found to have any significant influence on breast cancer risk. | ||
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+ | No effect was detected for spontaneous abortions when they were broken down (among parous women) by potential relationship with specific types of breast cancer. | ||
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+ | **__Unsuitable data handling.__** However, like many studies showing no effect for induced abortion on breast cancer risk, the Michels study contains many flaws. The statistical analysis section shows that the overall sample size includes more than 100,000 women. However, over 50,000 women neglected to complete the most important question on the baseline questionnaire, | ||
+ | \\ | ||
+ | \\ | ||
+ | \\ | ||
+ | This entry draws heavily from [[http:// | ||
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+ | Table 2 shows that the class of women with two or more induced abortions is fewer than 40 women. Said differently, | ||
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+ | **__Unsuitable comparisons.__** Additionally, | ||
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+ | **__No distinction between first- and second-trimester spontaneous abortions.__** Notably, the measures, and thus the analysis, do not distinguish between first- and second-trimester spontaneous abortions. | ||
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+ | **__Insufficient follow-up time.__** Significantly, | ||
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+ | **__Sampling bias (non-randomized sample).__** Furthermore, | ||
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+ | **__Health or survivor bias.__** Finally, women with a previous history of cancer or with a history of in situ breast cancers were excluded from the study. No explanation was given for the exclusion of in situ breast cancer. Though it is highly unlikely that all 4,065 previously-diagnosed and excluded cancers were cancers of the breast, it is likely that some were. Furthermore, | ||
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+ | =====6. 2008 Henderson Study (the California Teachers Study)===== | ||
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+ | The Henderson study104—another study based on a large, “gold standard” dataset (the California Teachers Study)—concluded that there was no increased risk of breast cancer with abortion. The Henderson study has many weaknesses, including unsuitable comparisons that mute the effect of induced abortion, the survivor or health bias produced by the exclusion of women with previous history of breast cancer and women with in situ breast cancer, sampling bias through the confinement of the study to educated women, and failure to distinguish between first- and second-trimester spontaneous abortions. | ||
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+ | This study assessed data collected for the California Teachers Study from 1995 to 2004, a nine-year period, on over 100,000 “current, recent, and retired female public school teachers and administrators.” | ||
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+ | **__Induced abortion.__** The Henderson study found no significant influence for induced abortion for either parous or nulliparous women when assessed generally or when assessed by the number of induced abortions procured, by age at first induced abortion, or by year of first induced abortion. | ||
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+ | **__Spontaneous abortion.__** The Henderson study found no significant influence for spontaneous abortion for either parous or nulliparous women when assessed generally, by the number of spontaneous abortions, or by age at first spontaneous abortion. | ||
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+ | **__Unsuitable comparisons.__** However (as is evident by the results described above), rather than comparing all cohorts against women with only complete, full-term pregnancies, | ||
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+ | **__Unsophisticated analysis of induced abortion.__** The second comparison was of parous women who had had only full-term pregnancies and parous women who had procured abortions. This a correct comparison, but the critical data on the sequence of births and abortions among the parous and aborting cohort are missing. (It may exist in the raw dataset, but it was not analyzed in the written journal article.) As noted earlier, the sequence of these reproductive events is extremely important in establishing the breast cancer risk abortion contributes. | ||
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+ | **__Health or survivor bias.__** Though the total number of women sampled who had an induced abortion is reported in the study, the time at which they had an abortion is not. All data regarding pregnancy history were collected by the time of the baseline questionnaire, | ||
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+ | **__Data not randomized.__** Like the Michels study, the Henderson study’s population sample is biased. Their sample is mostly white, and (as noted earlier) the IQ of teachers (i.e., of women with at least a Bachelor’s degree) is not representative of the general population. Henderson et al. control for race, but they do not control for education; hence, their results are not generalizable. The authors admit as much in their discussion: “The current results, may have limited generalizability. In addition to limited racial/ | ||
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+ | **__No distinction between first- and second-trimester spontaneous abortions.__** Finally, the Henderson study fails to distinguish between first- and second-trimester spontaneous abortions. This is a shortcoming. | ||
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+ | The presence of these biases are in line with an attitude expressed by one of the authors, Leslie Bernstein, in an interview in 2003, “I don’t want the issue relating induced abortion to breast cancer risk to be part of the mix of the discussion of induced abortion, its legality, its continued availability.” | ||
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+ | As an aside, the Henderson study states in its introduction that no studies that collected prospective data showed a link between abortion and breast cancer. In stating this, it disregarded the Howe study, a record linkage study (that is, a study that links medical records) not subject to the recall bias or reporting bias they suggest taints retrospective studies, that showed a significantly increased risk of breast cancer with induced abortion. | ||
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+ | =====7. 2013 Brauner Study===== | ||
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+ | A 2013 study by Braüner et al. of parous Danish women108 found no association between induced abortion and breast cancer risk. This prospective study included women identified through the Danish Diet, Cancer and Health study and assessed the effect of induced abortion with respect to the timing of one’s first full-term pregnancy. However, the study has several weaknesses that render it insignificant. | ||
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+ | Though this study is superior to other prospective studies in its methods and comparisons, | ||
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+ | **__Induced abortion.__** The Braüner study found no significant influence for induced abortion, generally or before or after a live birth, on breast cancer risk. | ||
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+ | **__Unsuitable data source.__** The study relied on data from the Diet, Cancer and Health study, which invited 79,729 women to participate. A mere 29,875 women accepted the invitation. Approximately 63 percent of participants chose to decline the invitation to participate in this study. | ||
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+ | **__Data not randomized.__** The authors also note that “[t]he rationale behind the study design was to include highly motivated people, and consequently secure a high participation in the follow-up investigation…. Not unlike other follow-up studies, the women who refused to participate had a low socioeconomic status. The participation was greater among women with a high income and a higher education compared to other Danish women (13). The incidence of breast cancer was also higher in the study population.”109 Despite including controls for education, Braüner et al. include no controls for socioeconomic status, and as Patrick Carroll states in his letter to the editor of the journal in which the Braüner study appears, the authors do not note how much higher was the incidence of breast cancer in the study population.110 Hence, as the sample was not representative of the general Danish population, its results may be imperfectly generalizable to the general Danish population. | ||
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+ | **__Health or survivor bias.__** Second, the study appears to be affected by the same sort of survivor bias or health bias that affects so many other studies. The authors excluded 337 women from their cohort who had previously experienced cancer. Though likely not all 337 cancer incidences were breast cancer cases, the importance of these cases’ exclusion becomes clear when one considers that the Braüner study only assessed 1,215 cases of breast cancer. Hence, up to 22 percent of breast cancers diagnosed within the cohort may have been excluded. This is a serious bias that would skew the results away from linkage of induced abortion and breast cancer. | ||
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+ | The error becomes all the more egregious in light of the age of the cohort in the Braüner study. The women included were aged 50 to 65 at the time of their inclusion in the Diet, Cancer and Health study between 1993 and 1997, and they were followed for an average of 12 years thereafter. Given that breast cancer from an induced abortion will most likely show up around a decade to 14 years thereafter, it is likely that only abortions procured about 10 to 14 years before the baseline period, when the women sampled were between the ages of about 36 and 55, would produce breast cancer detectable during the study period. However, females’ reproductive years are (approximately) between the ages of 15 and 45, and as demand for induced abortion among women over the age of 40 is relatively low,111 it may be that those breast cancers occurring as a result of earlier induced abortion were excluded. The breast cancers excluded may have been the only breast cancers caused by abortions—and thus precisely the breast cancers of interest to the study. | ||
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+ | **__Too-simple analysis of induced abortion.__** Third, the study does not assess the effect of repeated induced abortion. | ||
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+ | **__Restriction to parous women.__** Fourth, though the study utilizes the correct reference group (parous women with no abortion history), it restricts its analysis to parous women. Though the assessment of the effects of induced abortion on parous women is useful, there is also concern with the effect of induced abortion on nulliparous women, who never experience the protective benefit of full-term pregnancy. | ||
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+ | **__Omitted variable bias.__** Finally, the study neglects to include some important variables in its analysis. Its model is incomplete and does not include family history of breast cancer or age at menarche, for example, in its regressions. The Braüner study also does not include spontaneous abortion in its analyses, let alone distinguish between first- and second-trimester spontaneous abortions. | ||
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