It is widely recognised that bearing children reduces the risk of breast cancer, now scientists believe they have identified the source of this benefit.

Researchers at the University of Washington and Fred Hutchinson Cancer Research Centre have identified foetal cells “transplanted” to the mother before birth as the cause, in an article published this month in Cancer Research, journal of the American Association for Cancer Research.

The ability of cells from a growing foetus to take up long-term residence within its mother is a phenomenon called foetal microchimerism. According to the researchers, while foetal microchimerism has been implicated as a mechanism of autoimmune disease, it may also benefit mothers by putting the immune system on alert for malignant cells to destroy.

To test the idea, the researchers recruited 82 women, 35 of whom had been diagnosed with breast cancer. Approximately two-thirds of the women studied had had children, and more than half of the participants had given birth to at least one son. The researchers took blood samples from each participant and searched for male DNA, as they reasoned it is a relatively definitive matter to detect the male Y chromosome amid the mother’s native female cells within a blood sample.

Among the women with breast cancer, only five had male DNA in their bloodstream. Three of the five previously gave birth to sons, one had had an abortion and the other had never been knowingly pregnant. In total, about 14 percent of all women in the breast cancer group had male DNA in their bloodstream compared to 43 percent of women in the non-breast cancer group.

"Our research found that these persisting foetal cells may be giving a woman an edge against developing breast cancer,” said lead author Vijayakrishna K. Gadi, M.D., Ph.D., assistant professor at the University of Washington and research associate at the Fred Hutchinson Cancer Research Centre. “This experiment of nature is all the more fascinating because for years doctors treated a number of different cancers by transplanting cells from one person to another."

According to Dr. Gadi, these findings could provide a starting point for future research on the role of foetal microchimerism in the prevention of cancer. In addition, there are other reasons for male DNA to be in a woman’s peripheral blood, such as miscarriage and abortion, or possibly even blood transfusion or a male twin that was reabsorbed into the womb at an early stage of the pregnancy.