Chromosomes are genetic structures found in nearly every cell of our bodies. The first cell of an embryo is made up of a set of chromosomes from the sperm and another set from the egg. But if the cell doesn’t have the right number of chromosomes -  known as aneuploidy - this can lead to implantation failure, miscarriage or a baby affected with a genetic condition.


Pre-implantation genetic testing for aneuploidy (PGT-A), previously known as PGS testing, is a genetic screening test performed on embryos produced during an IVF treatment cycle. PGT-A gives information about your embryo's genetic health to help us select healthy embryos for transfer to improve your chance of achieving a successful pregnancy. 

Main benefits

  • PGT-A testing can help identify the embryo with the best chance of developing into a baby. PGT-A testing can’t change the number of viable embryos available for transfer but, by transferring only those that have the usual number of chromosomes, the time taken to establish a pregnancy can be shorter and the misery of failed cycles and risks of miscarriage can be reduced.

  • Where patients have undergone multiple rounds of IVF and have no explanation for the treatment failure, PGT-A testing can sometimes provide answers and help with decisions about future treatment options.


CARE’s own data shows that by performing PGT-A and transferring euploid embryos clinical pregnancy rates (CPR) are increased. In the year to 30 September 2019 CPR per embryo transferred across patients of all ages following PGT-A was 49% [1] compared with 38% [2] in non PGT-A cycles
  • [1] Based on 573 single per euploid embryo transferred resulting in 281 pregnancies with a foetal heartbeat.
  • [2] Based on 2645 per embryo transferred resulting in 1016 pregnancies with a foetal heartbeat 

Benefits exclusive to CARE patients

  • Time lapse imaging with CAREmaps is included. This provides a video of embryo development and information which may further enhance outcomes

Could PGT-A be right for me?

We might recommend PGT-A if you’re a woman in your mid–30s or older or you’ve suffered miscarriages, or had previous failed treatment; it could be that a chromosomal problem is the reason behind your failed treatments. Research suggests that more than 50% of human eggs have chromosomal problems and that this increases with age, so it’s thought to be the main reason older patients can struggle to start or grow their families.

We can use PGT-A to help us choose embryos with the right number of chromosomes for transfer, and reducing the misery of failed cycles and risk of miscarriage.


What is PGT-A and how is it performed at CARE?

Following fertilisation, an embryo divides into 2 cells, then four, eight, sixteen and so on, until 5 or 6 days after fertilisation there are over 100 cells.  At this stage the embryo is known as a blastocyst and it has an outer layer of cells called the trophoblast which surround the outside of the embryo. These are the cells that develop into the placenta. There is also a ball of cells inside the trophoblast called the inner cell mass and it is these cells that develop into the foetus and then baby.   In PGT-A, approximately 5 cells are removed from the outer layer of the blastocyst.  This procedure, called a biopsy, is performed by a highly skilled senior embryologist.  Biopsied embryos are then cryopreserved (frozen) and the biopsied cells are sent to a specialist laboratory for PGT-A analysis.

The results of the PGT-A test are available within a few weeks. If the biopsied cells have 46chromosomes, the embryo they came from is considered euploid and the transfer of these embryos can take place in a frozen embryo replacement cycle. Where the biopsied cells had fewer or more that 46 chromosomes, the embryo they came from is considered aneuploid and the embryos are not used in treatment. Occasionally, the results are more complex to interpret and, in these cases, CARE has access to expert genetic counselling services to help patients decide whether to transfer an embryo or not.

Studies have shown that removal of cells from the blastocyst doesn’t harm the embryo’s development and thousands of babies have been born after transfer of embryos that were biopsied.

What does PGT-A involve?

Usually our embryologists choose embryos for transfer based on the way they look and develop. This certainly does work, but PGT-A lets us go deeper, allowing us to analyse the genetic makeup of embryos and check that they have the right number of chromosomes before they’re transferred to the womb.

By using PGT-A our embryologists can single out embryos with abnormalities before transfer.

With PGT-A, each embryo will have one of four results


This means the embryo is genetically balanced with 46 chromosomes. These embryos can be transferred.


Aneuploid embryos aren’t genetically balanced and have too few, or too many, chromosomes. We won’t transfer these embryos, as they’re more likely to result in failed treatment or miscarriage.


Some embryos may have both chromosomally normal and abnormal cells or a mixture of the two – they are  known as mosaic embryos. The biopsy may take cells that are all abnormal or all normal. The presence of mosaicism can lead to false positive or false negative PGT-A results but is identified in fewer than 9% of cases.

No result

In less than 5% of cases, we simply won’t be able to tell whether the embryo is euploid or aneuploid. These can still be transferred or, in special circumstances, they might be re-biopsied.

The PGT-A process


Embryos are produced through an IVF cycle

Embryo biopsy

At 5-6 days after fertilisation embryos should reach the blastocyst stage. Blastocysts contain approximately 100 to 200 cells arranged into an inner cell mass and an outer ring of cells called the trophectoderm. The inner cell mass is made up of undifferentiated stem cells that will go on to form the foetus. The trophectoderm is a layer of cells that will become the placenta. A few trophectoderm cells are carefully removed by our embryologists and sent to the genetics laboratory for assessment, while we freeze your embryos and keep them safe.

Embryo freezing

We are very experienced in freezing and thawing of embryos, we use a process called ‘vitrification’. Vitrification involves much more rapid cooling than other less advanced methods of freezing, and it prevents ice crystals from forming. This preserves the quality of the cells and improves the chances of success when they are thawed. Vitrification has greatly improved the success of embryo freezing and it’s now a very reliable practice.


Testing is carried out using cutting-edge technology to analyse the number of chromosomes present within the embryo's cells.

Embryo transfer

Embryos which are identified as having the correct numbers of chromosomes and therefore most likely to result in a healthy baby are selected for transfer, or we can freeze your embryos for future use.

How effective is PGT-A?

The independent regulator of fertility treatment, the Human Fertilisation and Embryology Authority (HFEA), has developed a red-amber-green rating system to provide information about treatments that are offered on top of your routine fertility treatment known as treatment add-ons. They consider that the only way to be confident that a treatment is effective enough to be used routinely is to carry out a randomised controlled trial (RCT). In an RCT, patients are assigned randomly to two groups: a treatment group, given the new treatment and a control group, given either a well-tried treatment or a placebo.


The HFEA has given PGT-A a red rating because it considers that there is no evidence from such trials that PGT-A increases the overall chances of having a baby. There is information on the HFEA website about PGT-A that you may find useful to consider.


CARE did participate in a large scale RCT called the STAR trial, which was published in December 2019. This trial concluded that there was a significant increase in pregnancy rate in a subset of women aged 35–40 where they had more than two embryos available for testing,although overall in all age groups there was no increase in ongoing pregnancy. However, werecognise that that more robust clinical and laboratory trials are needed to prove whether or not PGT-A significantly increases live birth rates.


What does PGT-A cost?

There is an additional charge for PGT-A and for the transfer of any frozen euploid embryos, and ICSI. You can find information on the costs in your chosen CARE clinic’s fees schedule.


What are the risks of PGT-A and blastocyst biopsy

  • Although testing is generally very accurate, PGT-A testing may result in a false positive or false negative result in around 3% of cases. In the case of a false positive result, an embryo that may have implanted and developed may be discarded. In the case of a false negative result, an embryo with more or fewer than 46 chromosomes may be transferred.

  • It is very likely that following PGT-A testing there will be fewer embryos available for transfer or future use.

  • There is a small unquantified risk that removing cells from an embryo may affect development or cause changes during growth in the womb and lead to issues in later life.

Some embryos may have both chromosomally normal and abnormal cells or a mixture of the two – they are known as mosaic embryos.

The biopsy may take cells that are all abnormal or all normal. The presence of mosaicism can lead to false positive or false negative PGT-A results but is identified in fewer than 9% of cases.

  • As with any treatment there can be financial and emotional costs where treatment including PGT-A is not successful