Breast Cancer Risk Not Affected by Abortion or Miscarriage

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April 25, 2007 — Although a full-term pregnancy before the age of 35 years reduces the life-time risk for breast cancer, it appears that prematurely terminated pregnancies — whether by miscarriage or abortion — do not influence the risk for breast cancer. This conclusion, reported in a study published in the April 23 issue of the Archives of Internal Medicine, comes from an analysis of data from the Nurses Health Study II (NHSII).

This study, which is still ongoing, began in 1993 with 105,716 women who were aged 29 to 46 years when the study started. For the current analysis, a team of Harvard researchers headed by Karin B. Michels ScD, PhD, from Brigham and Women's Hospital, in Boston, Massachusetts, examined the data accumulated from 1993 to 2003. They investigated the association between induced or spontaneous abortion and the incidence of breast cancer, and found that there was none.

From all the study participants, 21% reported a history of miscarriage (21,753 women) and 15% reported a history of abortion (16,118 women). During the 10 years of follow-up, 1458 newly diagnosed cases of invasive breast cancer were found.

After adjustment for established breast cancer risks, the incidence of breast cancer was no higher among those women who had had an abortion (hazard ratio , 1.01; 95% confidence interval [CI], 0.88 - 1.17) and was slightly reduced in those who had experienced a miscarriage (HR, 0.89; 95% CI, 0.78 - 1.01).

These findings tie in with those of some previous reports, but not all, the researchers comment. In studies that have examined induced abortion separately from miscarriage, about half found an increase in the risk for breast cancer, while the other half found no association. Most studies of miscarriage have found no association.

In 2003, an international expert panel convened by the National Cancer Institute concluded that induced abortion is not associated with an increase in breast cancer risk. These data from the NHSII "provide further evidence of a lack of an important overall association between induced or spontaneous abortions and risk of breast cancer," Dr. Michels and colleagues conclude.

However, the authors also highlight the difficulties in obtaining accurate data on such a sensitive personal issue as induced abortion and comment that underreporting is probable, especially for abortions that occurred before 1973, when abortions were illegal in the United States. A general underreporting would lead to an underestimation of the true association, they point out. The proportion of women reporting a history of abortion in this study (15%) is similar to the rates of 16% and 19% reported from 2 European studies.

However, none of these studies included all the reproductive years of the women participating, "thus leaving room for misclassification," the researchers write. Also, these rates contrast with the estimate from the Alan Guttmacher Institute that 1 (25%) in 4 women younger than 45 years of age in the United States has had at least 1 induced abortion. "The nurses in our cohort may not be representative of the social classes most likely to use abortion services," the authors of the current study comment.

Arch Intern Med. 2007;167:814-820.