Today, more than 20 years later, that same principle of pre-implantation genetic diagnosis (PGD) has now been applied to screening IVF embryos for any type of chromosomal disorder in a bid to spot the defects and transfer only healthy embryos with a likely chance of implanting as a pregnancy.
Success rates for PGD
However, what seems logical in principle has not yet proved successful in practice. The IVF patients who seem most affected by chromosome problems are those in an older age group and those with a history of spontaneous abortion. But studies in both these patient groups have shown no benefit in those whose embryos were screened by PGD. Indeed, one study even found that pregnancy rates in those having PGS were lower than in those having conventional IVF and ICSI (without PGS).
As a result of this and other studies, the British Fertility Society recently issued less than enthusiastic guidelines about PGS, stating that 'at present patients should be informed that there is no robust evidence that PGS for advanced maternal age improves live birth rate per cycle started'.
Comparative genomic hybridsation (CGH)
All the studies proving so disappointing in PGS used a chromosome investigation method which could analyse no more than nine chromosomes - when the full complement in an embryo is made up of 23 pairs. As a result, concerns arose that the problem with PGS was not so much the logic but more the technology for the analysis. That problem, however, may now have been solved by the introduction of a new technique which can screen all 23 pairs of chromosomes. And it's a technique - known as comparative genomic hybridisation, or CGH - used by the London Women’s Clinic.
When pilot studies of the CGH technique were performed in poor prognosis IVF patients in the USA, results were described as 'absolutely phenomenal'. One study in 45 infertile couples, with an average maternal age of 38 years and at least one previous unsuccessful IVF cycle, reported a pregnancy rate of more than 80%- and a 70% probability that each screened and transferred embryo would progress to a viable pregnancy.
This technique, like all the earlier versions, requires the biopsy of a single cell from an embryo (in this case after five days of culture growth) and its complex chromosomal analysis. But because this analysis takes several days to complete, the embryo must be frozen until a 'diagnosis' is available and a 'healthy' embryo is ready for transfer. Another CGH technique is able to perform the analysis in 'real time' (though from a very early fertilised egg) and so does not require embryo freezing before transfer.
The future of CGH
There are many questions still to be answered about embryo selection with PGS, one of which is the central question of whether CGH will prove more effective than the earlier technologies. But many other questions still remain - when best to perform the biopsy, whether the analysis of a single cell accurately reflects the full chromosomal, and what effect will freezing have on the screened embryo.
The need to transfer fewer embryos to reduce multiple pregnancy rates has put added pressure on embryo selection and the quest to find the one embryo most likely to implant in the womb and become a pregnancy. If, as studies suggest, the likelihood of implantation is most affected by genetic factors inherited from the egg, then an analysis of the full chromosomal complement of the embryo - as CGH allows - may well prove one step nearer to that holy grail.
This article was first featured in 'Ova' the magazine of the London Women’s Clinic. For your free copy of this magazine please visit the London Women’s Clinic’s website www.londonwomensclinic.com.
About The London Women’s Clinic
The London Women’s Clinic provides a range of fertility treatments in centres across the UK to assist couples and individuals experiencing fertility problems. Its team of consultants and nursing staff has over 20 years' experience of diagnosing and treating fertility problems. It offers a full range of diagnostic and treatment programmes for both male and female fertility disorders.