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 serious condition.


Pre-implantation genetic testing for aneuploidy is a genetic screening test performed on embryos produced during an IVF treatment cycle. PGT-A gives information about your embryos’ genetic health to help us select the best embryo for transfer and improve your chance of achieving a successful pregnancy. 

What does PGT-A involve?

Usually our embryologists choose the best embryos 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. This improves IVF success rates for some patients and reduces both the chance of multiple pregnancies and the likelihood of miscarriage.

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

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

  • Aneuploid

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.

  • Mosaic

Mosaic embryos have some cells that are euploid and some that are aneuploid. Depending on the chromosomes affected, we might still be able to transfer these embryos. If you have mosaic embryos we’ll offer you an appointment with a genetics counsellor to discuss your options further.

  • 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.

Could PGT-A be right for me?

If you’ve had treatment before that hasn’t worked, you’ve suffered miscarriages, or if you’re a woman in your mid–30s or older, we might recommend PGT-A; 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 women can struggle to start or grow their families.

During your next cycle, we can use PGT-A to help us choose embryos with the right number of chromosomes for transfer, which could give you a better chance of success.

Patients in their first cycle who have suffered miscarriage previously or who are older than 35 may also wish to consider PGT-A.

Pre-implantation genetic testing has the potential to:

  • Improve IVF birth rate per embryo transferred
  • Minimise the incidence of miscarriages and birth defects caused by irregularity in the chromosome number of eggs
  • Reduce the incidence of multiple pregnancies whilst maintaining a high live birth rate

The PGT-A process

Embryos are produced through an IVF cylce
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.

Are there any risks associated with PGT-A

Performing a PGT-A biopsy on embryos at Blastocyst stage has little or no effect on successful development. This is because an embryo developed to Blastocyst stage has many more cells, which can be extracted without affecting an embryo’s potential for pregnancy. The necessary freezing and thawing of embryos is highly successful with over 95% survival rate after thawing. There is therefore, a small risk that the embryo will not survive the process.

HFEA Comment

What is the evidence for PGT-A?

Large high quality research is not yet available to provide clear evidence of the success of PGT-A. It is important to understand that PGT-A will not increase your overall chances of having a baby, although it may shorten your time to pregnancy by helping us select the best embryo sooner. It may help to reduce the likelihood of having a miscarriage, but more evidence is needed to confirm these findings. When considering whether to have PGT-A, you should also think about the cost. For more information, please see the HFEA website or read our FAQs.