The scientists believe their research could help treat infertility

A University of Cambridge team has discovered a molecule, STAT3, that helps embryos implant in the womb.

They say a drug that blocks STAT3 should act as a contraceptive without disrupting hormones, or raising the risk of blood clots, as the Pill does.

Details are published in Proceedings of the National Academy of Sciences.

The Cambridge team has shown that STAT3 plays a key role in signalling to the cells lining the womb that they should allow the embryo to attach.

Without that signal an embryo would not attach to the womb wall, and a pregnancy would be impossible.

Role in tumours

Drug companies are already researching ways to block STAT3 because it has also been implicated in the formation of tumours, and diabetes.

CONTRACEPTIVE PILL Associated with higher risk of: Breast cancer High blood pressure Blood clots Associated with lower risk of: Ovarian cancer Endometrial cancer Acne

Lead researcher Dr Andrew Sharkey said the latest research suggested the experimental compounds under development might also act as a contraceptive.

"These compounds may be effective at preventing implantation in humans if appropriate dose and delivery systems can be devised to target the uterus, such as gels."

The researchers are also investigating whether deficiencies in STAT3 signalling might be responsible for certain types of infertility.

They believe it might be possible to treat some cases of infertility by activating STAT3 signalling.

Dr Anna Glasier, director of family planning for Lothian Primary Care NHS Trust, said molecules which had a specific effect on the body were the most likely to minimise the risk of side effects.

Risks and benefits

But she said modern contraceptive pills, although associated with side effects, including an apparent increased risk of breast cancer, also had beneficial effects.

For instance, the combined pill is associated with a decreased risk of ovarian and endometrial cancers, and can help to improve skin conditions, such as acne.

"If you get more and more specific in the molecules that are used to provide contraception, then you will lose all those non-contraceptive benefits," she said.

Toni Belfield, of the Family Planning Association, said: "All research into improving women and men's choices in contraception is to be welcomed.

"But what is just as important is to improve the information that we provide about current contraceptive methods, to remove the many myths and misinformation about their risks and benefits."

The research was funded by the World Health Organization as part of a programme to identify new targets for contraception development.