Hormones released during pregnancy raise levels of a protein that blocks the growth of breast cancer, scientists have said.

Although the research is at an early stage they believe it could lead to promising new treatments.

It is well known that pregnancy hormones such as oestrogen, progesterone and human chorionic gonadotropin reduce the risk of breast cancer. However, the reason why they have this effect is not clearly understood.

Part of the answer may be that the hormones trigger the production of a protective protein, a new study suggests.

Alpha-fetoprotein (AFP) is normally generated in the liver and the foetal yolk sac, which supplies an unborn baby with its first nutrients.

Scientists in the US conducted experiments with rats at high risk of mammary cancer.

The animals were treated with oestrogen alone, oestrogen plus progesterone, or human chorionic gonadotropin.

In a large proportion of the rats, the hormones prevented or delayed the development of mammary cancers, and each treatment also raised blood levels of AFP. When laboratory cultures of breast cancer cells were exposed to AFP, their growth was blocked. The scientists believe this suggests it is the protein that is responsible for the protective effect of pregnancy hormones.

Lead researcher Dr Herbert Jacobson, from Albany Medical College in New York, said: "Hormones in pregnancy, such as oestrogen, all induce AFP, which directly inhibits the growth of breast cancer.

"The body has a natural defence system against breast cancer. AFP needs to be safely harnessed and developed into a drug that can be used to protect women from breast cancer."

The findings are published today in the journal 'Cancer Prevention Research'. Professor Powel Brown, an editorial board member at the journal, pointed out there was no direct evidence of AFP having an anti-cancer effect, and that mammary tumour growth was not blocked in all the rats.

"This study is promising and suggests that additional animal studies need to be done before translation to humans," said Prof Brown.

Dr Jacobson's team is now investigating how the AFP molecule might be converted into a breast cancer preventative agent.