Neither spontaneous nor induced abortion appear to be related to breast cancer risk in premenopausal women, according to a retrospective study of data from the Nurses' Health Study.
“The association between abortion and breast cancer has previously been considered in numerous studies,” with conflicting results, Karin B. Michels, Ph.D., of Harvard Medical School, Boston, and her associates wrote. About half of the studies of induced abortion have found an increased risk of breast cancer, while the rest have found either no association or an inverse association. Most studies of spontaneous abortion have found no link with breast cancer risk.
The investigators reviewed about a decade's worth of relevant data from the Nurses' Health Study (NHS) and reported their results in the Archives of Internal Medicine.
The NHS is an ongoing cohort study of the associations between lifestyle factors, reproductive factors, and breast cancer and other major illnesses. The first biennial NHS survey in 1989 involved more than 100,000 female registered nurses aged 25–42 years in 14 states.
Information on induced and spontaneous abortions was first obtained in 1993. Dr. Michels and her associates assessed breast cancer incidence in 105,716 of the subjects who provided that information and were followed for approximately 10 years. Of these women, 15% reported that they had had one or more induced abortions and 21% reported that they had had one or more spontaneous abortions.
A total of 1,458 newly diagnosed cases of invasive breast cancer developed during follow-up. There was no association between induced abortion and breast cancer risk. There was a suggestion of an inverse association between spontaneous abortion and risk, but that may have been because of chance, the investigators said (Arch. Intern. Med. 2007;167:814–20).
“Breast cancer cases in our study population were almost exclusively premenopausal; therefore, our results may not be generalizable to postmenopausal women,” they noted.
In 2003, the summary report of a National Cancer Institute international expert panel concluded that the evidence to that date showed no link between induced abortion and breast cancer risk.