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Neither induced abortion nor miscarriage increases the risk of breast cancer, a large prospective US study has found.
Results of previous case control and retrospective studies have been inconsistent. Anti-abortion groups in the United States have claimed that having an abortion increased a woman's risk of breast cancer by 30%, and anti-abortion counsellors used the argument to dissuade women from having an abortion.
The researchers, from Harvard University, say that about a quarter of US women aged under 45 years have had at least one induced abortion (Archives of Internal Medicine 2007;167:814-20).
They wrote, “In this cohort of young women, we found no association between induced abortion and breast cancer incidence.” They found no relation between the incidence of breast cancer and number of abortions, age at which the woman had an abortion, whether she had had a previous pregnancy, and the time between a previous pregnancy and an abortion.
The researchers found that among women who had had one or more induced abortions the hazard ratio for having breast cancer was 1 (95% confidence interval 0.9 to 1.2), after they adjusted for known risk factors for breast cancer. Among women who had had one or more spontaneous abortions the hazard ratio for breast cancer was 0.9 (0.8 to 1). Breast cancers in the study group were mostly in premenopausal women.
The study looked at the association between breast cancer and induced or spontaneous abortions in 105716 registered nurses who were aged 25 to 42 at the beginning of the study and were free of cancer.
Previous studies had retrospectively asked women with breast cancer if they had had an abortion. The current study's lead author, Karen Michels of Brigham and Women's Hospital and Harvard Medical School in Boston, told the Reuters news agency that women with breast cancer, searching for reasons for their disease, are more likely than other women to report having had an abortion.
Information in the study was collected anonymously at the outset and updated every two years. However, women may have under-reported abortions, which were illegal in the United States until 1973, during part of their reproductive years. The women were followed up from 1993 to 2003.
Overall 15% of the nurses said they had had one or more abortions, and 21% said they had had one or more spontaneous miscarriages. Of the women in the study 1458 (about 1.4%) developed breast cancer. The researchers reviewed medical records and found that breast cancer was histologically confirmed for 99% of the women reporting breast cancer whose records could be found.
The authors say that a full term pregnancy before the age of 35 reduces a woman's lifetime risk of breast cancer, perhaps by speeding up breast cell differentiation. They wrote, “An incomplete pregnancy may not result in sufficient differentiation to counter the high levels of pregnancy hormones that may foster proliferation. However, the biological mechanisms are uncertain, and a prematurely terminated pregnancy may not affect breast cancer risk at all.”
In 2003 the US National Cancer Institute convened an expert panel to review the evidence for an association between induced or spontaneous abortion and the risk of breast cancer. The panel concluded that no link existed.