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Drugs used in IVF for older women may increase their risk of having a baby with Down's syndrome, experts say.

Doctors already know that the chance of having a baby with the genetic condition goes up with the age of the mother, especially for those over 35.

Now UK researchers, who looked at 34 couples, think drugs used to kick-start ovaries for IVF in older women disturb the genetic material of the eggs.

Work is now needed to confirm their suspicions, a meeting in Sweden heard.

And they do not yet know the magnitude of risk, but say it could also cause many other genetic conditions, not just Down's.

The findings, presented at the European Society of Human Reproduction and Embryology's annual conference, come from a UK study of 34 couples undergoing fertility treatment.

All of the women in the group were older than 31 and had been given drugs to make their ovaries release eggs ready for their IVF treatment.

When the researchers studied these now fertilised eggs they found some had genetic errors.

These errors could either cause the pregnancy to fail or mean the baby would be born with a genetic disease.

A closer look at 100 of the faulty eggs revealed that many of the errors involved a duplication of coiled genetic material, known as a chromosome.

Often, the error resulted in an extra copy of chromosome 21, which causes Down's syndrome.

But unlike "classic" Down's syndrome which is often seen in the babies of older women who conceive naturally, the pattern of genetic errors leading to Down's in the IVF eggs was different and more complex.

And this led the researchers to believe that it was the fertility treatment that was to blame.

Lead researcher Professor Alan Handyside, director of the London Bridge Fertility, Gynaecology and Genetics Centre, said more research was now needed.

"This could mean that the stimulation of the ovaries is causing some of these errors. We already know that these fertility drugs can have a similar effect in laboratory studies. But we need more work to confirm our findings."

If more tests back up their suspicions then it would mean that doctors should be more cautious about using these treatments, he said.

The researchers believe their work could also help identify which women might be better off using donor eggs for IVF instead.

Co-investigator Professor Joep Geraedts, of Bonn University in Germany, said: "This in itself is already a big step forward that will aid couples hoping for a healthy pregnancy and birth to be able to achieve one."

UK fertility expert Mr Stuart Lavery said: "There's a huge increase in the number of women undergoing IVF at later ages as people delay the age of starting a family.

"Previously we have always thought that these chromosomal abnormalities were related to the age of the egg.

"What this work shows is that a lot of the chromosomal abnormalities are not those that are conventionally age-related. It raises the concern that some of the abnormalities might be treatment-related.

"It's a little unclear as to whether it's the medication itself that is affecting the egg quality or whether it's the medication that is just forcing the issue and allowing eggs that nature's quality control system would have otherwise excluded, to arise."