While they may continue to produce eggs throughout their 30s and 40s, the reservoir of potential eggs from which they are taken has shrunk to almost nothing, it suggests.
As the body chooses the best eggs from the reserve, the likelihood is that the quality of the eggs will suffer as you get older increasing the difficulty of conception and the risk of an unhealthy baby.
The new research by the Univeristy of St Andrews and Edinburgh University is the first to colate the actual decline of the “ovarian reserve” – the potential number of eggs women are born with – from conception to the menopause.
It shows that on average women are born with 300,000 potential egg cells but this pool declines at a much faster rate than first thought.
By the age of 30 there is only 12 per cent left on average and by the age of 40 just three per cent.
Dr Hamish Wallace, the co-author, said: “Our research shows that they are generally over-estimating their fertility prospects.
“Our model shows that for 95 per cent of women, by the age of 30 years, only 12% of their maximum ovarian reserve is present, and by the age of 40 years only three per cent remains.”
The researchers said many women make the mistake of thinking that because they are still producing eggs that their fertility remains constant. But this new research shows that it delcines rapidly.
The study collected together information from 325 different women in the UK, US and Europe all at different ages and assessed their reserves of potential eggs.
They then used the information to make a graph of the potential decline in the ovarian reserve throughout the life of the average woman.
Dr Wallace, at Edinburgh’s Royal Hospital for Sick Children, said the research could help predict which women have early menopause and when to freeze eggs from ovarian cancer victims.
The research published in the journal Public Library of Science One also showed that there was an enormous difference between the size of individual women’s “ovarian reserve”.
Some women had more than two million, whereas others had as little as 35,000.
The research is the latest to warn women that they must not leave it too late to conceive.
Women’s fertility declines substantially after her mid-thirties but the speed of the drop differs for each individual and many face heartache when they find they have left it too late.
Some doctors have called for regular fertility screening in the same way women are screened for cervical cancer.
