Neither spontaneous nor elective abortion appears to be associated with risk for breast cancer in premenopausal women, according to an article in the April 23 issue of Archives of Internal Medicine.

In contrast, Women younger than age 35 years who carry a pregnancy to term appear to have a reduced lifetime risk of breast cancer, according to background information in the article.

Karin B. Michels, ScD, PhD, and colleagues at Brigham and Women's Hospital, Harvard Medical School and Harvard School of Public Health, Boston, examined the association between abortion and breast cancer in 105,716 women who were part of the Nurses' Health Study II (NHSII). The women were between age 29 and 46 years at the beginning of the study in 1993.

In 1993, and again every two years through 2003, they answered questions about whether and at what age they had had miscarriages or elective abortions and provided information about breast cancer risk factors and diagnoses.

A total of 16,118 participants (15 percent) reported history of elective abortion and 21,753 (21 percent) had a history of spontaneous abortion. Between 1993 and 2003, 1,458 new cases of breast cancer occurred among the women.

"In this cohort study of young women, we found no association between induced abortion and breast cancer incidence and a suggestion of an inverse association between spontaneous abortion and breast cancer incidence during 10 years of follow-up," the authors wrote.

"We observed associations in two subgroups, an association between induced abortion and progesterone receptor-negative breast cancer and an inverse association between spontaneous abortion before the age of 20 years and breast cancer incidence," they continued. However, they cautioned that these secondary analyses are based on small numbers of women. "No obvious mechanisms can be provided for these subgroup findings; thus, chance has to be considered as a possible explanation."

A 2003 international expert panel convened by the National Cancer Institute reviewed and assessed research regarding reproductive events and the risk of breast cancer and concluded that based on existing evidence, induced abortion is not associated with an increased risk of breast cancer.

"The 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," the authors concluded. "Among this predominantly premenopausal population, neither induced nor spontaneous abortion was associated with the incidence of breast cancer."