Eventually, we might find some non-toxic method of giving some combination of hormones or drugs to women that would give them some resistance to cancer

Bert O'Malley, Baylor College of Medicine

Hormones released during pregnancy trigger the changes in the breast tissue of young women. The research raises the possibility of one day developing a pill that could protect against breast cancer.

In experiments, rats and mice given doses of natural hormones that mimic pregnancy were protected against breast cancer.

When the animals were injected with a known carcinogen, they did not develop as many mammary tumours as expected.

Cancer 'resistance'

The research was carried out by a team at Baylor College of Medicine, Houston, Texas, US.

Further work might eventually lead to a treatment that would safeguard women against breast cancer, said Bert O'Malley, who led the research.

Statistics show that age of first pregnancy influences breast cancer risk

He told BBC News Online: "Eventually, we might find some non-toxic method of giving some combination of hormones or drugs to women that would give them some resistance to cancer."

A breast cancer "pill" was a possibility, he said, but such an advance was some years away.

About 10% of women in the West will develop breast cancer at some stage of their lives. Epidemiological evidence has long pointed to a link between pregnancy in early life and a lower risk of breast cancer - although the reason why was not clear.

Experiments in animals show that pregnancy hormones influence a gene called P53 that produces a tumour suppressor protein. The same is thought to happen in people.

In young women, whose breast tissue is still developing, this appears to have life-long effects. The effect is most marked in women who give birth before they are 20, said Professor O'Malley.

The research is published in the journal Proceedings of the National Academy of Sciences.