Younger couples who believe they have fertility problems should not be dismissed by their GPs

Mr Richard Kennedy, Walsgrave Hospital

Lead author Dr David Dunson said that, to his knowledge, this was the first study to spot the tell-tale signs of falling fertility at such young ages.

He said: "Although we noted a decline in female fertility in the late 20s, what we found was a decrease in the probability of becoming pregnant per menstrual cycle, not in the probability of eventually achieving a pregnancy."

The finding partly supports what many experts already believe - that a woman's fertility gradually diminishes throughout her 20s after peaking in her late teens.

Falling success

Evidence from IVF treatments suggest that success rates begin to gradually tail off from the start of the 30s, falling more dramatically from the mid to late 30s.

A leading fertility specialist says that young couples who are having problems conceiving need to be aware that the chances of successful IVF treatment, should that prove necessary, are better if the woman is in her 20s rather than her 30s.

Mr Richard Kennedy, from Walsgrave Hospital near Coventry, said: "Some health authorities refuse to pay for couples to undergo IVF until they are at least 30, which is crazy, because not only do the couple have to wait for years, but the chances of success are falling.

"Younger couples who believe they have fertility problems should not be dismissed by their GPs."

On the positive side, however, Dr Dunson, from the National Institute of Environmental Health Sciences, found that the so-called fertile window - days in the cycle on which a woman can conceive, did not shorten regardless of the woman's age, remaining at approximately six days.

His findings were published in the journal Human Reproduction.