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One cycle of IVF, one egg, one embryo and one baby.

It sounds simple, and yet for the 35,000 women who undergo IVF in the UK each year it's unlikely.

Only one in three IVF cycles is successful, each one costing potentially thousands of pounds.

But a new chromosome counting technique pioneered at Oxford University could put paid to all that.

CGH (comparative genomic hybridisation) is used to check for any significant abnormalities present in the chromosomes of the embryo before implantation in IVF treatment.

A small number of cells are removed from the growing embryo five days after fertilisation and the DNA in them is scanned for any clear problems.

The results are available within 24 hours, which allows the maximum information to be obtained from the embryo before it is used.

Armed with this knowledge, doctors can then ensure that only embryos with the correct number of chromosomes are transferred in IVF, thereby improving the chances of a successful pregnancy and reducing the likelihood of miscarriage or Down's syndrome.

Advantageous

Dr Dagan Wells developed the technique of applying CGH to five-day-old embryos (called blastocysts) at the Nuffield Department of Obstetrics and Gynaecology in Oxford.

He says: "Five days after the egg is fertilised it has more than 100 cells so we can safely take some cells for testing. The tests have worked really well, giving an accuracy rating of 98% - and there is no impact on the embryo of taking these cells either."

The chromosome screening technique is not intended to look for any specific genetic disorders relevant to the couple being treated or anything more subtle than viability.

"We are only testing for ability to be alive," says Dr Wells.

This gets around the two problems of IVF - the number of faulty embryos produced in a typical IVF cycle and the problem of distinguishing between the faulty and healthy embryos using traditional checks.

Embryos can seem to be growing well under a microscope but may still have chromosome abnormalities, and it's these abnormalities which lead to miscarriage or a Down's syndrome baby at full term.

The CGH technique can also be used at an earlier stage on patients' eggs, rather than on the embryos - created when the sperm has fertilised the egg.

This is particularly advantageous for women who have fewer, good embryos to start with because they will not have to wait until the blastocyst stage - five days after fertilisation - for the embryos to be analysed and transferred.

In some IVF patients - particularly those above the age of 42 - it can become more and more difficult to find 'normal' embryos, because fewer eggs and fewer embryos are produced as women age.

Studies analysing the benefits of CGH to date are promising, but experts agree that more scientific evidence is required before the technique is applied routinely in IVF clinics.

A study of American women in Colorado found that pregnancy rates increased by more than 50% following chromosome screening, but since it is normal to transfer around three embryos during IVF in the US rather than just one in the UK, this enhances the results.

For Dr Wells, it is the implantation rate that is important - the chance that one embryo can make a pregnancy.

Stewart Lavery, consultant gynaecologist and director of the IVF at Hammersmith Hospital in London, has been carrying out CGH for a year.

"It's a really exciting technique which does have potential, but we have to be cautious. We need some good evidence first," he says.

The safety of the technique is certainly not in question. What IVF experts need is evidence of which patients will benefit from CGH and by how much.

Mr Lavery says: "We think it will help slightly older women who have an increased risk of a Down's syndrome baby and also young women who have a repeated failure of implantation."

Failure to conceive

So it may yet be a long time before it's available on the NHS. But it could make financial sense if it saved on costly IVF cycles, avoided terminations and cut the numbers of miscarriages dealt with in hospital.

What is certain is that in the search for a 'normal' egg and a 'normal' embryo IVF doctors have to use their judgement. Is there such a thing as a 'normal' embryo?

Even for women not going through IVF, becoming pregnant is a very tricky and complicated business. Failure to conceive is common, occurring regularly even before the woman is aware of it.

"We have already beaten the odds just by being alive," says Dr Wells.

For women going through IVF, anything that increases the odds of a baby is all that matters.