Induced Abortion and Breast Cancer. The Scientific Evidence

Reprinted 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 non-significant 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 favourable 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 from conclusive, 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.)