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The study suggests that women age 45 or younger who have had induced abortions have a relative risk of 1.5 (50 percent increased risk) for breast cancer compared to women who had been pregnant but never had an induced abortion.
In epidemiologic research, relative risks of less than 2 are considered small and are usually difficult to interpret. Such increases may be due to chance, statistical bias, or effects of confounding factors that are sometimes not evident. In an editorial accompanying the study, Lynn Rosenberg, Sc.D., Boston University School of Medicine, points out that a "difference in risk of 50 percent (relative risk of 1.5) is small in epidemiologic terms [human population studies], and severely challenges our ability to distinguish whether it reflects cause and effect or whether it simply reflects bias."
Rosenberg notes that "the overall results as well as the particulars are far from conclusive, and it is difficult to see how they will be informative to the public."
Daling and colleagues did not find a consistent pattern of increasing or decreasing risk associated with age at abortion, as would be expected by many scientists. [Risk was greater for women who had their first induced abortion before age 18 (relative risk of 2.5) and for women who were 30 years of age or older (relative risk of 2.1).] Furthermore, the risk did not vary by number of abortions, whether abortion preceded or followed a full-term pregnancy, or by length of time to diagnosis of breast cancer. One key point is that women aged 45 or younger who had miscarriages were not found to be at increased risk for breast cancer.
Taken together, the inconsistencies and scarcity of existing research do not permit scientific conclusions.
In the Daling study, the researchers analyzed data on 845 white women who were diagnosed with invasive or in situ breast cancer from 1983 to 1990 and 961 control subjects. All the women were born after 1944. Data were collected on reproductive history, family history of breast and other cancers, and lifestyle and other factors. The study population was from three counties in Washington State. Only white women were included in the study because of the small minority population in this area.
The researchers also found that risk for breast cancer was more enhanced for women having an induced abortion prior to age 18 if their pregnancy was interrupted during the 9-to-24-week period of gestation. However, this finding was based on small numbers.
Studies published in the "JNCI" are peer-reviewed by scientists and represent the views of the authors. Papers published in the journal do not necessarily reflect the views held by NCI or any other component of the federal government.
Date Last Modified: 12/94
The Referred To Article
Re: Breast CancerReprinted from the January 1995 issue of Medical Sciences Bulletin, published by Pharmaceutical Information Associates, Ltd.
It is estimated that, as of 1990, one of every four US women under the age of 45 years had had an induced abortion. Evidence from clinical and animal studies has been interpreted as indicating that induced abortions increase the risk of breast cancer. Animal models suggest that full-term pregnancies protect against breast cancer because differentiation of breast stem cells, which makes them less susceptible to carcinogens, occurs late in pregnancy. Interruption of a pregnancy results in proliferation of breast cells without the possible protective effect of differentiation. A recent epidemiologic study supports the hypothesis of increased breast cancer risk after induced abortion. Because the study has been widely reported in the print and broadcast news, and because of the strong emotional and political aspects of both induced abortion and breast cancer, it is important to understand just what the new report states scientifically. J. R. Daling et al., of the Fred Hutchinson Cancer Research Center and the University of Washington, Seattle, conducted an interview-based case-control study of women younger than 45 years. They compared responses from 845 women who had invasive or in situ breast cancer those from with 961 women from the general population. The findings showed a greater risk of breast cancer for women who reported one or more induced abortions than for women who had carried their fetuses to term. The risk varied, in a nonsignificant manner for subgroups defined by age at the time of the abortion, length of gestation, lactation in later pregnancies, and family history of breast cancer. The findings did not indicate variations in risk according to whether the abortion preceded or followed a full-term pregnancy. Spontaneous abortion did not carry an increased risk. (This may be because 56.3% of women with spontaneous abortions nursed a child during the 5 years after the spontaneous abortion, compared with only 14.3% of women who underwent induced abortions.) This study determined that women who underwent induced abortions had a 50% greater risk of breast cancer than women who did not have abortions. When abortion was induced during the last month of the first trimester, the risk almost doubled. For women younger than 18 years at the time of a first induced abortion, the relative risk was 2.5. This was reduced to 1.3 if the abortion was induced earlier in gestation. Women older than 30 years at the time of a first induced abortion had a relative risk of 2.1. (Daling JR, et al. J Natl Cancer Inst. 1994; 86: 1584-1592.) Commenting on the report by Daling et al., Lynn Rosenberg of the Boston University School of Medicine noted that a number of studies have produced contradictory conclusions regarding a link between induced abortions and breast cancer risk. She stated, "A typical difference in risk (50%) is small in epidemiologic terms and severely challenges our ability to distinguish if it reflects cause and effect or if it simply reflects bias." Induced abortions are generally underreported. If the women with breast cancer-who were more willing than healthy women to participate in this study-were also more willing to report induced abortions, bias would have occurred. In addition, those women reporting induced abortions had less favorable risk profiles (parity, age at first term pregnancy, and lactation) than women reporting spontaneous abortions. Rosenberg also noted that the studies done so far have looked only at surgically induced abortions. RU486 and other abortifacient drugs, should they ever be approved in the United States, would have to be considered separately. Of the Daling study, Rosenberg commented, "... the overall results as well as the particulars are far fromonclusive, and it is difficult to see how they will be informative to the public. Whatever future results show, the decision to continue or terminate an unplanned pregnancy will still need to be based on a balanced consideration of the entire range of relevant issues-personal ethical considerations, the desire for a child, the ability to care for it, and the total health implications of continued pregnancy versus induced abortion." (Rosenberg L. J Natl Cancer Inst. 1994; 86: 1569-1570.) |