Pregnancies ending in abortion do not increase the risk of developing breast cancer

March 25, 2004

Results of a major international collaboration investigating the relationship between abortion and breast cancer are published in this week's issue of THE LANCET. Authors of the study conclude that the totality of the worldwide evidence does not suggest any increase in the risk of developing breast cancer for women who have had a pregnancy that ended in miscarriage or induced abortion.

The Collaborative Group on Hormonal Factors in Breast Cancer (secretariat based at the University of Oxford, UK) re-analysed data from 53 epidemiological studies undertaken in 16 countries.

Data were available from 44000 women who had taken part in studies where any history of abortion had been recorded before-and often many years before-any breast cancer was diagnosed. Results were expressed as the "relative risk" of breast cancer, which compares the chances of developing breast cancer in women with, and without, some such record of abortion; a relative risk of 1.0, or less than 1.0, would indicate no adverse effect on the subsequent risk of breast cancer. In these studies, the averaged relative risk of breast cancer was 0.98 for women who had had a pregnancy that ended as a miscarriage and 0.93 for women who had had a pregnancy that ended as an induced abortion, indicating no increased risk of breast cancer after miscarriage or abortion. The number of abortions was also not associated with any change in breast-cancer risk.

Data on 39000 women with breast cancer in potentially less reliable studies (where women were asked after the diagnosis of breast cancer about any previous abortions) gave, on average, misleading results. Professor Richard Peto says "Studies can give misleading results if women are asked about previous abortions only after they are diagnosed with breast cancer. This may well be because, on average, women with breast cancer are more likely than other women to disclose any prior induced abortions." (quote by e-mail, does not appear in published paper).

Professor Valerie Beral comments: "The totality of the worldwide epidemiological evidence indicates that pregnancies ended by induced abortion do not have adverse effects on women's subsequent risk of developing breast cancer."
Contact: Cancer Research UK Press Office, 61 Lincoln's Inn Fields, London WC2A 3PX, UK; T)44-20-7061-8300; (out of hours:44-7050-264059); E)


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