Pre-implantation genetic screening (PGS)
On this page:
- What is PGS?
- Is PGS for me?
- How does PGS work?
- What is my chance of having a baby with PGS?
- What are the risks of PGS?
- Where do I start?
What is pre-implantation genetic screening (PGS)?
PGS (also known as aneuploidy screening) involves checking the chromosomes of embryos conceived by in vitro fertilisation (IVF) or intra-cytoplasmic sperm injection (ICSI) for common abnormalities. This avoids having abnormal embryos transferred to the womb during IVF or ICSI.
Chromosomal abnormalities are a major cause of the failure of embryos to implant, and of miscarriages. They can also cause conditions such as Down’s syndrome.
Is PGS for me?
Your clinic may recommend genetic screening of embryos, particularly if:
- you are over 35 and have a higher risk of having a baby with a chromosome problem (such as Down’s syndrome)
- you have a family history of chromosome problems
- you have a history of recurrent miscarriages
- you have had several unsuccessful cycles of IVF where embryos have been transferred
- your partner’s sperm is known to be at high risk of having chromosome problems.
However, various studies have questioned whether or not PGS is effective at increasing the chance of having a live birth.
How does PGS work?
The procedure for PGS is likely to be as follows:
Step 1. You undergo normal IVF treatment to collect and fertilise your eggs.
Step 2. The embryo is grown in the laboratory for two - three days until the cells have divided and the embryo consists of about eight cells.
Step 3. A trained embryologist removes one or two of the cells (blastomeres) from the embryo.
Step 4. The chromosomes are examined to see how many there are and whether they are normal.
Step 5. One, two or three of the embryos without abnormal numbers of chromosomes are transferred to the womb so that they can develop. Any remaining unaffected embryos can be frozen for later use.
However, embryos that have been biopsied may not be suitable for cryopreservation and use in subsequent treatment cycles.
Step 6. Those embryos that had abnormal chromosomes are allowed to perish or may be used for research (with your consent).
Possible variations to this procedure
Testing at five – six days
It is possible that instead of removing and testing one or two cells from a two – three day old embryo, some centres may allow the embryo to develop to five - six days, when there are 100-150 cells.
More cells can be removed at this stage without compromising the viability of the embryo, possibly leading to a more accurate test.
Alternatively some centres may test eggs for chromosomal abnormalities before they are used to create embryos. Polar bodies (small cells extruded by eggs as they mature) can be extracted and tested.
Comparative Genomic Hybridisation (CGH)
A small number of clinics are now using a procedure called comparative genomic hybridisation (CGH) which allows centres to test for abnormalities in all 23 chromosomes.
These abnormalities may or may not be of biological significance, but their presence will lower the chance of finding suitable embryos for transfer.
What is my chance of having a baby with PGS?
Because a large proportion of patients who receive PGS are older patients, patients with a history of miscarriages or other indications and also because many of the embryos produced are not suitable for transfer to the womb, the success rate varies considerably depending on the patient’s individual circumstances.
The average success rate for PGS treatment in the UK for in the year from 01/01/2008 - 31/12/2008 is:
- ** (2/7) for women aged under 35
- ** (2/15) for women aged between 35-37
- ** (7/25) for women aged between 38-39
- ** (12/42) for women aged between 40-42
- ** (1/29) for women aged between 43-44
- ** (0/12) for women aged over 44
** Percentages are not calculated where there are less than 50 cycles. Figures given in brackets are (cycles resulting in a live birth / all cycles started).
Various studies have questioned whether or not PGS is effective at increasing the chance of having a live birth. There is a lack of evidence that having a treatment cycle with PGS will increase your chances of having a baby compared to having a treatment cycle without PGS.
More robust randomised controlled trials are needed before a decision can be made either way.
Centres are required to validate the use of PGS (i.e demonstrate there is evidence) for each category of patients they offer it to (e.g advanced maternal age, recurrent implantation failure, recurrent pregnancy loss and male factor infertility).
What are the risks of PGS?
Some of the risks involved in PGS treatment are similar to those for conventional IVF. For more information, see:
Other problems unique to PGS treatment include:
- some embryos may be damaged by the process of cell removal
- possibility that no embryos are suitable for transfer to the womb after PGS.
It is important to understand that there is no guarantee against a miscarriage occurring even though PGS has been carried out prior to embryo transfer.
Where do I start?
To find clinics offering PGS, use the advanced search in our Find a clinic database:
You can also access a list of Genetic Centres and Services in your area by visiting the Genetic Alliance UK.
If you are considering this treatment, you should talk to your GP to go through the options available. Your GP can also refer you to see a specialist at your local hospital or fertility clinic.
PGS relies on the theory that all the cells in a human embryo are chromosomally identical, so that if you examine one cell from an embryo, it will show whether or not all the other cells have a chromosomal abnormality.
However, research has shown that in some embryos (known as mosaic embryos), the cells are not chromosomally identical. As a result, many such embryos will be discarded that are in fact capable of producing a normal pregnancy.
Page last updated: 26 June 2009