Neither Abortion Nor Miscarriage Associated With Breast Cancer Risk

Women younger than age 35 who carry a pregnancy to term appear to have a reduced lifetime risk of breast cancer, according to background information in the article. Pregnancy may accelerate breast cell differentiation, the process by which cells take on specialized roles. "An incomplete pregnancy may not result in sufficient differentiation to counter the high levels of pregnancy hormones that may foster proliferation," the rapid growth and division typical of cancer cells, the authors write. "However, these biological mechanisms are uncertain, and a prematurely terminated pregnancy may not affect breast cancer risk at all."

Karin B. Michels, Sc.D., Ph.D., 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 at the beginning of the study in 1993. At that time, and again every two years through 2003, they answered questions about whether and at what age they had had miscarriages or induced abortions and provided information about breast cancer risk factors and diagnoses.

A total of 16,118 participants (15 percent) reported having a history of induced 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 write.

"We observed associations in two subgroups, an association between induced abortion and progesterone receptor--negative breast cancer [cancer that does not respond to the hormone progesterone] and an inverse association between spontaneous abortion before the age of 20 years and breast cancer incidence," they continue. However, they caution 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 conclude. "Among this predominantly pre-menopausal population, neither induced nor spontaneous abortion was associated with the incidence of breast cancer."

The NHSII is supported by a Public Health Service grant from the National Cancer Institute, National Institutes of Health, U.S. Department of Health and Human Services.