An international collaboration led by Oxford University concluded in last week's issue of The Lancet that there is no increased risk of breast cancer for women who have had a pregnancy ending in miscarriage or abortion.
It had previously been thought that there might be a link between the risk of breast cancer and pregnancies that had ended in miscarriage or abortion, but the researchers' review of existing data concluded that no such link existed.
The Collaborative Group on Hormonal Factors in Breast Cancer, a secretariat based at the University, re-analysed data from 53 epidemiological studies undertaken in 16 countries. Data were available from 44,000 women who had taken part in studies where any history of abortion had been recorded 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 39,000 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, which may have led to the suggestion that a link existed. 'Studies can give misleading results if women are asked about previous abortions only after they are diagnosed with breast cancer,' said Professor Richard Peto. 'This may well be because, on average, women with breast cancer are more likely than other women to disclose any prior induced abortions.'
Professor Valerie Beral said: '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.'