Pre-implantation genetic screening for aneuploidy (PGS)

This is another form of genetic testing which is carried out to check for chromosomal abnormalities in the embryos. This means that only healthy embryos will be transferred into the uterus.

What is ‘aneuploidy?’

It is a genetic condition in which the embryo has an abnormal number of chromosomes. Basically, a normal healthy person has 23 pairs of chromosomes (or 46 individual chromosomes) in their genetic make-up. Half of these chromosomes are from their mother and the other half are from their father.

An egg and sperm contain 23 chromosomes and when they are fertilised the chromosomes mix together.

As the embryo develops, its chromosomes double in number during a process called cell division and the cell divides into two.

But, there are a few cases where this goes wrong. The chromosomes fail to divide evenly which results in an extra chromosome in one of the cells. Cell division continues but this one cell is now carrying the extra chromosome which causes a genetic defect.

Older women are at more risk of developing a chromosomal abnormality. Once a woman reaches 35 she is looking at a 3 fold increase in the risk of chromosomal abnormalities.

This is where PGS is useful. It can be used to screen every embryo in a woman aged 35 and over to ensure that only normal embryos are implanted.

The reason behind this is that every woman has a small number of abnormal eggs. However, this number increases over time so that an older woman has a greater chance of producing an abnormal embryo than a younger woman.

In fact, a woman of 40 may have a 50% chance of an abnormal embryo.

With PGS, abnormal embryos are terminated and healthy ones transferred which means fewer embryos are implanted. This also reduces the risk of multiple births.

Both PGD and PGS are seen as useful techniques in that they can screen out potentially damaging diseases. Plus it is seen as less risky than the standard amniocentesis test. This test if often carried out during pregnancy which carries a greater risk to the unborn baby. If the test shows a genetic disorder, for example Down’s syndrome then there is a painful decision to be made. This is much more difficult than deciding upon the future of an embryo that has undergone PGD/PGS screening.

Pre-implantation Genetic Diagnosis (PGD) Guide Index:

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• Pre-implantation Genetic Diagnosis (PGD)
• Ethical and moral issues with PGD
• Who is suitable for PGD?

• Is PGD effective?
• The PGD procedure
• Pre-implantation genetic screening for aneuploidy

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