WASHINGTON: Having children could reduce the risk of getting breast cancer because cells with strong protective characteristics are transferred from the baby in the womb to the mother, a study has showed.

Researchers in Seattle studied 82 women, 35 of whom had been diagnosed with breast cancer, to test a theory that fetal cells which take up residence in the mother, called fetal micro-chimerism, protect against breast cancer.

"Most studies have looked at autoimmune diseases where chimerism has been shown to be bad, but so many women harbor micro-chimerism after pregnancy in detectable levels that I reasoned there must be some reason why nature decided this must be a good thing to do," the lead author on the study, V K Gadi said.

"Perhaps it's this function of clearing cancer cells from your body. Another possibility is that it could participate in tissue repair," he said.

The research team at the University of Washington and Fred Hutchison Cancer Research Center in Seattle searched blood samples from each participant for male DNA, which was easier to isolate than DNA from a female child.

"I don't have any intrinsic reason to believe that a male child would be any more protective than a female child," Gadi stressed, adding that the researchers hope to conduct studies into fetal chimerism from all sources.

The results showed that about 14 per cent of all women in the breast cancer group had male DNA in their bloodstream compared with 43 per cent of women in the non-breast cancer group.

"My hypothesis was that may be fetal cells can get into a mother and recognise a pre-cancer breast cancer cell and kill it before it becomes an active cancer,"