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Pre-implantation genetic screening (PGS or PGT-A)

What is PGS?

PGS (also known as aneuploidy screening) is a treatment add-on. It involves checking the chromosomes of embryos created by in vitro fertilisation (IVF) or intracytoplasmic sperm injection (ICSI) for abnormalities. PGS doesn’t check for  specific genetic diagnoses.

During PGS, a single cell or a small number of cells is removed from the embryo. The DNA of these cells is then tested to see whether they have any chromosomal abnormalities. Only embryos without chromosomal abnormalities are placed back in the womb.

Who might be recommended to have PGS?

Older women are more likely to have eggs with the wrong number of chromosomes, so traditionally it’s been offered to women over 37. Abnormal chromosomes are thought to be the main reason why older women have difficulties conceiving and are more likely to have a miscarriage or a baby with Down’s Syndrome.

Your doctor may also recommend PGS if you have a family history of chromosome problems, if your sperm is at risk of carrying abnormal chromosomes, or if you’ve had several miscarriages or failed IVF attempts without explanation.

However, there is limited evidence to show that PGS benefits these groups (see more below.) It’s also not a guaranteed way to prevent miscarriage or to have a baby.

What causes abnormal chromosomes?

When the cells of eggs, sperm and embryos divide this can lead to too many, or too few, chromosomes, or with missing or added pieces of DNA. Abnormal chromosomes happen very frequently during the normal production of eggs, sperm and during embryo development.

Does PGS work?

In the past, some clinics recommended PGS to older women with a history of miscarriage or failed IVF cycles or people with a family history of chromosome problems. However, to date there is little evidence showing it improves success rates for these groups and therefore it is considered a treatment add-on.

Also, research has shown that often the cells in an embryo are not chromosomally identical (called mosaicism). PGS relies on test results from one or a small number of cells being representative of the embryo as a whole. As this is not always the case, it may be possible for an embryo to give an abnormal test result when it is in fact capable of producing a healthy pregnancy.

Until larger trials have been run and we have more evidence, there’s no guarantee that PGS can improve your chances of a successful pregnancy.



There’s a risk of misdiagnosing a healthy embryo

How safe is it for the embryos?

PGS involves taking a biopsy, to remove a cell or number of cells, from an embryo which can sometimes cause damage to the embryo and prevent it from developing once it has been transferred into the womb.

Are there any other risks?

PGS carries the same risks as IVF – find out more about the risks of fertility treatment.

However, PGS also has some treatment-specific risks. This includes the possibility of a misdiagnosis (eg due to mosaicism described above). There is also the risk that, if all the embryos are found to have abnormal chromosomes, there won’t be any embryos to put back in the womb. This is especially likely for older women.

Can I have PGS on the NHS?

No, PGS is not currently funded by the NHS. PGS can be expensive so you should talk to your clinic about likely costs and make sure your clinician clearly explains why PGS might be appropriate for you. 


Review date: 24 August 2022

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