A "genetic contraceptive" that could interfere with the make-up of women's eggs during ovulation each month and which is designed to have none of the side effects of traditional hormone based pills is being developed by scientists.
The new approach, the researchers say, should make it impossible for sperm to fertilise women's eggs. If the technique can be made to work it will mark the first serious advance in chemical contraception since the first stages in the development of oral contraception - work begun by Carl Djerassi nearly 60 years ago which led to the pill.
Because a genetic contraceptive will not flood the body with sex hormones it will avoid many of the side effects caused by conventional birth control pills, the researchers believe.
Traditional oral contraceptives, which are based on the steroids progesterone and oestrogen, have been linked to an increased risk of blood clots and certain cancers, and may also affect mood and libido, and cause weight gain.
The new contraceptive relies on a technique called RNA interference, which uses tiny fragments of genetic material to block the activity of genes in the body. The process is so powerful it was lauded as a revolution in medical science last year, when two American scientists, Andrew Fire and Craig Mello, won the Nobel prize for their work on it.
Zev Williams and his team at Brigham and Women's Hospital in Boston, part of Harvard Medical School, use RNA interference to block a gene called ZP3 that only appears in eggs when they are being ovulated. Normally, the gene produces a protein that coats the outside of the egg, the zona pellucida, and is vital for sperm to latch on to to achieve fertilisation.
To test the principle Dr Williams first bred mice that had the ZP3 gene knocked out. The animals developed healthily but were completely infertile.
In further tests the team showed they could use RNA interference to block 95% of ZP3 activity in human cells temporarily.
The research was announced at the annual meeeting of the American Society for Reproductive Medicine in Washington yesterday.
"If you could block this in women you could prevent pregnancy from occurring," said Dr Williams. "For women who use the pill just as a contraceptive, a non-hormonal approach would be wonderful. You could get all the benefits without the nausea, the headaches, the mood alterations, and the raised risk of thrombosis, stroke and heart attacks."
The scientists hope to begin full tests on animals within five years, and if they consider the method works safely a product, probably in the form of a suppository or skin patch, could be available within 10 years. It would be unlikely to replace traditional contraceptive pills entirely as many women use those tablets to ease period pains or make their menstrual cycles more regular.
Bill Ledger, a fertility specialist at Sheffield University, said: "A lot of women have side effects on the contraceptive pill and so are anxious about the long-term effects. All types of contraception have their downsides and no one has yet invented something that is perfect.
"This is a new concept ... if it were available, I am sure a lot of people would want to take it."
Andrew Sharkey, a senior research associate at Cambridge University, said: "The oral pill is nearly 60 years old and there's been no real advance since then. If you target the egg you don't get fertilisation or produce an embryo or interfere with a pregnancy in any way, and for some people that's more ethically acceptable."
Ashley Grossman, a consultant endocrinologist at Bart's hospital in London, was more sceptical, mentioning the "huge amount of [research] effort going into male contraception" and adding: "The young people I meet are increasingly ... going to mechanical barriers such as condoms."