Study Frays Abortion, Cancer Link

A study of unprecedented scope indicates that abortion does not increase a woman's risk of breast cancer -- except, perhaps, for a small group of women who have abortions in the second trimester of pregnancy.

 Danish investigators arrived at this finding through an analysis encompassing all of the 1.5 million women born in Denmark between 1935 and 1978. Of these women, 280,965 had abortions and 10,246 developed breast cancer. Most of the abortions were performed between the 7th and 14th weeks of pregnancy, as they are in the United States.

 The researchers say the study wields enough statistical power to dispel any lingering scientific uncertainties about abortion's impact on breast cancer risk. "There simply is no risk for the majority of women," asserts team member Mads Melbye of Statens Serum Institute in Copenhagen.

 For the almost 5,000 women who had an abortion less than 7 weeks into a pregnancy, the breast cancer risk was 20 percent lower than that for the group as a whole. For the nearly 850 women who had abortions after 18 weeks, however, the risk increased significantly. Fourteen of these women developed breast cancer -- nearly twice as many as would have been expected. Melbye says this detail, which his team is now trying to explain, is the most intriguing to emerge from the study.

 The finding may represent a statistical anomaly, or the women in this group may have some factor in common, other than abortion, that raises their cancer risk. Or the heightened risk in this cluster of women may lend limited support to the general hypothesis that women who interrupt their pregnancies are more prone to breast cancer.

 That hypothesis, which originally led scientists to study abortion and breast cancer, was based on two observations. The first is that women who have had a baby have a lower risk of breast cancer. The second is that the flood of estrogen early in pregnancy causes a proliferation of breast cells that do not mature until late in pregnancy. This suggests that the breasts of women who have had abortions remain laden with immature cells that may be vulnerable to cancer-causing influences.

 Efforts to test the theory yielded a cascade of studies, all with inconclusive results. Chief among these is a combined analysis, or metanalysis, of 23 studies suggesting that women who had abortions increased their risk of breast cancer by 90 percent.

 Many earlier studies were hampered by their small size. The rest, including those used in the metanalysis, calculated breast cancer risk by comparing women who said they had had abortions with those who said they had not.

 Studies have shown, however, that many women do not accurately reveal such sensitive information. This was the fatal flaw in the metanalysis, Melbye says. "A metanalysis can never be better than the quality of the individual studies."

 The Danish researchers relied solely on doctors' reports. This was possible because, for half a century, Danish doctors have been required to report both the abortions they perform and the breast cancer cases they diagnose. This requirement solved another problem -- population size. The study includes five times as many women who have had abortions or breast cancer as any similar study done in the United States, notes Patricia Hartge of the National Cancer Institute in Bethesda, Md.

 By creating subgroups of women based on such factors as the week of pregnancy during which an abortion was performed, the researchers could draw fine distinctions between relative breast cancer risks.

 Hartge says the study's size and detail "strengthen the credibility of the new findings. The study thus provides important new evidence to resolve a controversy that previous investigations have been unable to settle."