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THURSDAY, March 25 (HealthDayNews) -- An analysis of existing data from more than a dozen different countries has found that women who have had an abortion are not at an increased risk of developing breast cancer.

"Pregnancies that end in induced abortion do not increase the risk of developing breast cancer," says Sir Richard Peto, a professor of medical statistics at the University of Oxford and one of the authors of the study appearing in the March 27 issue of The Lancet. "That really has been proved beyond a reasonable doubt. There is no increase in the subsequent risk of developing breast cancer. We've got very good evidence of safety in that respect. It is a definite answer to this particular question."

"I think that their review is very convincing," adds Dr. Paul Tartter, senior attending physician in surgery at St. Luke's-Roosevelt Hospital Center and an associate professor of surgery at Columbia University College of Physicians and Surgeons, both in New York City.

While previous research has shown women who have full-term pregnancies have a lower risk of developing breast cancer, the collective research on women who have had pregnancies ending in miscarriage or abortion was often contradictory.

To try to answer the question once and for all, the Collaborative Group on Hormonal Factors in Breast Cancer assessed the collective worldwide epidemiological evidence on the subject. The group was established in 1992 to look at the global epidemiological evidence on the relationship between breast cancer and factors that affect female hormones.

For this paper, researchers brought together data from 53 epidemiological studies conducted in 16 countries.

In all, data were available for 44,000 women who had participated in various prospective studies -- studies where the record of whether women had had an abortion was made before any diagnosis of breast cancer. Women who had had a pregnancy ending in abortion had no increased risk of developing breast cancer in comparison with otherwise similar women.

"I think it's pretty clear that if you take all the studies, both positive and negative, there is really no significant effect of abortion on breast cancer," says Tartter.

The researchers reviewed separately data on an additional 39,000 women who had breast cancer in retrospective studies -- studies where women who had already got breast cancer were asked whether they had ever had an abortion. They found the results from such studies were unreliable. In these cases, each woman was asked about previous abortions only after already receiving a diagnosis of breast cancer. "She is then more likely to tell you things to help your research," says Peto, a pioneer in the field of meta-analysis, in which experts reach conclusions by reviewing many large studies.

The type of research methodology used made a big difference in the reliability of the results.

"They separated studies in which breast cancer patients were asked if they ever had an abortion vs. studies where those who ever had an abortion were followed," Tartter explains. "The more valued is the study which follows the women."

"Prospective studies get a reliable answer," Peto says. "We showed that, on average, retrospective studies got the wrong answer."

More information

For more on breast cancer, visit the U.S. National Cancer Institute. This Web site has more on the work of the Collaborative Group.

(SOURCES: Sir Richard Peto, F.R.S., professor, medical statistics, University of Oxford, England; Paul Tartter, M.D., senior attending physician, surgery, St. Luke's-Roosevelt Hospital Center, and associate professor, surgery, Columbia University College of Physicians and Surgeons, New York City; March 27, 2004, The Lancet)

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