Preimplantation genetic testing is performed by assessing a small part of an egg, embryo or blastocyst to determine its genetic make-up. The test is performed before implantation to ensure the embryo has a balanced number of chromosomes or determine whether the embryo has a specific genetic sequence that is linked to a known genetic disorder.

Preimplantation genetic testing, or preimplantation genetic testing for aneuploidy (PGT or PGT-A), is a test that checks for abnormalities in the number of chromosomes in embryos. It has also previously been known as preimplantation genetic screening (PGS). 

Embryos with an abnormal number of chromosomes, which are also known as aneuploid embryos, have a reduced chance of developing into a baby. If they do, there is a chance that the baby may be born with a genetic condition as a result. PGT is used to identify aneuploid embryos, as these embryos are generally considered unsuitable for fertility treatment procedures. 

In order to carry out the procedure, an embryologist will remove several cells from a well-developed blastocyst embryo. These will then be tested for any sign of chromosomal abnormalities. When the removal is done carefully, the embryo should still be able to form a viable pregnancy with the majority of cells that remain.

Most typically, PGT will be offered as an add-on or additional procedure to:

  • Biologically female patients over the age of 37
  • Patients who have experienced several miscarriages
  • Patients who have experienced several failed IVF cycles
  • Patients with a family history of chromosomal problems
  • Biologically male patients whose sperm may carry abnormal chromosomes

PGT does not carry any known additional risks to patients undergoing fertility treatment. However, it is known to carry some risks for the embryo involved in the procedure:

  • There is a chance that the test may give the wrong result, even though most PGT techniques are now very accurate
  • Choosing to use PGT as part of your treatment may mean fewer embryos are available to use in the treatment itself or to freeze for further treatments in the future
  • Removing cells from an embryo may damage it and prevent it from developing
  • If performed incorrectly, removing cells from the embryo may cause changes in the later growth of the embryo.

Most PGT procedures will now be carried out at the blastocyst stage of development, on Day 5 or Day 6 after the egg has been fertilised in a laboratory environment. 

Currently, preimplantation genetic testing is only considered an additional treatment by the Human Fertilisation and Embryology Authority (HFEA). Under its traffic light system, PGT is also listed as red. This is because there is no evidence from randomised controlled trials (RCTs) that PGT carried out at the blastocyst stage is significantly likely to improve a patient’s chances of having a baby through IVF.  What it does do, however, is highlight any normal (euploid) embryos more likely to lead to a viable pregnancy, thereby reducing the number of treatment cycles, time to pregnancy and time to live birth in patients who do end up with euploid embryos available for transfer.

It is possible that a reduced number of embryos for biopsy following the procedure may counter any benefit gained from the procedure itself.  In other words, if there are only one or two embryos available in a treatment cycle, the chances of pregnancy by transferring both embryos as opposed to biopsying both embryos would be the same, the only difference being the ability to find out whether the embryo is chromosomally normal or not prior to transfer.  Embryos known to contain a majority of abnormal cells cannot be selected for embryo transfer.