"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 (miscarriage) and breast cancer incidence during 10 years of follow-up," says a team from Brigham and Women's Hospital, Harvard Medical School and the Harvard School of Public Health, Boston.

They reported their findings in the April 23 issue of the Archives of Internal Medicine.

The researchers examined data on almost 106,000 women who took part in the Nurses' Health Study II, which began in 1993. The women were ages 29 to 46 at the start of the study.

Among the women, more than 16,000 reported having had an induced abortion at some point in their lives and almost 22,000 reported having had a miscarriage. Between 1993 and 2003, there were 1,458 new cases of breast cancer diagnosed among the women.

They found no link between abortion, miscarriage and breast cancer generally. However, "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."

The researchers noted that these secondary analyses were based on small numbers of women, however. "No obvious mechanisms can be provided for these subgroup findings; thus, chance has to be considered as a possible explanation," they wrote.