A contraceptive drug that avoids the side effects of hormonal birth control is on the horizon, say scientists.
New method blocks sperm entering the egg
An American Society for Reproductive Medicine conference was told a technique called "RNA interference" could stop sperm entering the egg.
Oral contraceptives can cause nausea, headaches, and low sex drive and raise slightly the risk of DVT and strokes.
However, the new "Pill" is at least a decade away - and may have its own side-effects.
RNA interference is a way of "silencing a gene" to stop it working properly, and the researchers from Brigham and Women's Hospital in Boston have identified a gene called ZP3 which is active in eggs just before they are fertilised.
ZP3 produces a protein which allows the sperm to bind to the surface of the egg. If this protein isn't there, the egg can't be fertilised.
The Boston team "silenced" the ZP3 gene in mice, and found they could not get pregnant.
Dr Zev Williams, who presented the research to the conference, said: "We simply don't have a contraceptive drug that is non-hormonal and reversible.
"What we are trying to do is to think about contraception in a new way.
"Obviously there are going to be hurdles and it is going to take a lot of time, but the need is there and we think it can be achieved."
Professor Bill Ledger, from the University of Sheffield, said that a lot of women still had side effects, even on the modern hormonal contraceptives. "This is a new concept. If it were available, I am sure a lot of people would want to take it.
"If it isn't hormonal, that's a big selling point."
However, the researchers estimate that it will be at least 10 years before clinical trials of an RNAi contraceptive would be possible.
Dr Martin Fabani, a researcher in the technique at Cambridge University, said that obstacles would need to be overcome, and that there was no guarantee that side-effects could be avoided completely.
He said: "RNA interference is fantastic and there was a big hype around it, but people are starting to see what we call 'off-target' effects - where the therapy has an unwanted effect elsewhere in the body.
"Every single application has some degree of off target effects."
The research into ZP3 has one advantage in this respect, as the gene appears to be active only in eggs prior to the moment of fertilisation, and nowhere else in the body.
The researchers say this means it could be "switched off" without necessarily affecting either the prior development of the egg and ovulation, or other parts of the body.