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The lack of credible evidence for reporting bias notwithstanding, conclusive proof about the abortion-breast cancer link--as nearly all researchers agree--can come only from studies using prospective data like the 1989 New York study that found a 90 percent increased risk of breast cancer attributable to induced abortion. This was the importance of the "definitive" Danish computer-registry study led by Dr. Mads Melbye and published in the January 1997 New England Journal of Medicine. The Melbye study claims to be definitive not only because of the prospective nature of the data, but also because of its size, encompassing all 1.5 million women born in Denmark between 1935 and 1978, over 280,000 of whom have had legal abortions, and over 10,000 of whom have had breast cancer. The study concludes that "Induced abortions have no overall effect on the risk of breast cancer," having found an overall risk increase associated with abortion of exactly 0 percent.
The study falls apart, however, upon the close scrutiny made possible by the substantial body of published data concerning the same population of Danish women. Although abortions have been legal in Denmark since 1939, the Melbye study used computerized abortion records beginning only with 1973. The authors understate this weakness of the study, acknowledging only that "we might have obtained an incomplete history of induced abortions for some of the oldest women in the cohort." But a check of pre-1973 abortions shows that they misclassified some 60,000 women who had abortions as not having had any.
Yet even this egregious misclassification is not the most significant flaw in the study. The generally long latency of breast cancer means that the study largely compared younger women (with more abortions and fewer incidents of breast cancer) to older women (with more incidents of breast cancer and fewer abortions). The authors are aware of this potential source of error. But in correcting for it by adjusting for a "cohort effect" is the acknowledged fact that the incidence of breast cancer has been generally rising for most of this century. The problem, however, is that the causes of this rising incidence are unknown, and since the frequency of induced abortion has similarly risen through most of this century, abortion may well be a cause of the cohort effect. And if abortion is indeed a factor in the risk of breast cancer, the cohort adjustment the Melbye study performs necessarily eliminates its effect--making the 0 percent increased risk a virtually guaranteed result.
And there is plenty of evidence that induced abortion is indeed the missing cohort factor. First, Melbye and his colleagues show enough data to compute the unadjusted relative risk, and this calculation shows a 44 percent risk increase (it is extremely disturbing from a scientific point of view, that this number did not appear in the paper). Second , a 1988 study of part of the same cohort of Danish women found a 191 percent increased risk among childless women (the only women reported on) who had any induced abortion. Third, a close examination of the legal abortion in Denmark since 1939 shows a striking parallel with the rates of breast cancer incidence. The abortion rate peaked in 1975 and the average age at which a Danish woman had an abortion is twenty-seven, which means that the greatest number of abortions were performed on women born around 1948. But the latest age-specific data in Denmark show that the incidence of breast cancer is maximal for women born between 1945 and 1950, and is on the decline for women born more recently. A proper analysis would likely show a significant breast cancer increase in the neighborhood of 100 percent for induced abortion.
Abortion is not a controversial subject in Denmark and Dr. Melbye seems a sincere and competent man. But his study reveals the entrenched bias in favor of the view that abortion is harmless to women, a bias that is decades old. One in every six Danish women has had at least one abortion, which means that complicity in abortion decisions is pervasive in she society. How willing can members of such a society be to acknowledge that they have put themselves and those they love at risk of one of the most dreaded, life-threatening diseases that a woman can get? What hope is there in such a society for scientific integrity to overcome a witting or unwitting wall of denial?
Fortunately, abortion is still a controversial subject in America, but denial from high places of its harmfulness to women is hard to miss--even in the partial funding for the Melbye study provided by the U.S. Department of Defense. If we are to maintain scientific integrity in medical research, we must denounce wherever it appears the manipulation of studies to provide socially desired results. The point of maintaining scientific integrity in medicine, of course, is not just to preserve the abstract notion of truth, but to save the lives of both women and their babies.
JOEL BRIND is Professor of Biology and Endocrinology of Baruch College of the City University of New York and Editor of the new Abortion-Breast Cancer Quarterly Update
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