Preimplantation genetic screening

Preimplantation genetic screening (PGS) is used to screen embryos for common aneuploidies by techniques such as FISH. Initially five chromosomes were screened (13,18,21, x and y) and now more commonly seven and colour FISH is used (13,16,18, 21, 22, x and y). As techniques progress then more chromosomes can be looked at, although there can be a problem with error rates, as you have to rehy-bridize the blastomere. The way forward is likely to be techniques that can look at the whole 23 pairs of chromosomes, such as whole genomic amplification and the use of gene chip technology.

INDICATIONS

At present the recommended indications for PGS are:

• Recurrent miscarriages

• Recurrent IVF failures

• Patients over the age of 37 undergoing IVF

• Previous aneuploid pregnancy

BENEFITS

If these techniques are shown to be clinically successful, then the obvious benefits would be improvement in overall IVF success rates, reduction in miscarriage rates and a reduction in terminations of pregnancies for aneuploidies such as Downs Syndrome.

PROBLEMS

At present there is not a robust enough evidence base to recommend PGS and more research needs to be done. Most importantly the use of techniques that can screen more of the chromosomes will be developed to make PGS more likely to be clinically beneficial. Surprisingly, the safety aspects so far appear to be very reassuring and even though potentially a quarter of the embryo is removed this does not seem to impact on the overall implantation rate and health of the offspring. Again, longer-term follow-up is required to prove conclusively the safety of these techniques.

They are also relatively expensive and require dedicated laboratory facilities, particularly if specific gene defects are looked for.

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