"It is well established that pregnancies that end in a full-term birth ultimately confer a protective effect on breast cancer risk," Dr. Gillian K. Reeves, of the University of Oxford, UK, and colleagues write. "The effect of incomplete pregnancies on the risk of breast cancer has been less clear."

    The researchers examined the role of abortion on breast cancer risk among 267,361 women enrolled in the European Prospective Investigation into Cancer and Nutrition between 1992 and 2000. The data came from 20 centers across nine countries.

    A total of 4,805 women were diagnosed with breast cancer during a follow-up that averaged 6.6 years. The researchers included all pregnancies that ended prior to 20 weeks or the stage of viability -- spontaneous abortions or miscarriages, and therapeutic or induced abortions.

    "Overall, almost two thirds of women reported never having had any type of abortion, while about one third reported having had at least one type of abortion," Reeves and colleagues report.

    About 20 percent reported having a spontaneous abortion compared with about 16 percent who reported having an induced abortion. Only 3.8 percent of women reported having both types of abortion.

    Having one spontaneous abortion did not affect the risk of breast cancer and having two or more spontaneous abortions only slightly increased the risk. No evidence of a relationship between one or more induced abortions and breast cancer was found.

    "Overall, the findings provide further unbiased evidence of the lack of an adverse effect of induced abortion on breast cancer risk," the team concludes.

    (Agencies)