What's the Difference Between PGT-A, PGT-M and PGT-SR?

The three tests differ in what they look at. PGT-A counts the chromosomes in an embryo to check there are 46. PGT-M looks for a specific single-gene condition that the parents are known to carry, such as cystic fibrosis or sickle cell anemia. PGT-SR looks for structural problems in the chromosomes, such as a translocation, in couples known to have one. All three are carried out on embryos created through IVF, using the same biopsy procedure, and they answer different questions rather than being alternatives to one another.

What is PGT-A and who is it for?

PGT-A, preimplantation genetic testing for aneuploidy, counts the number of chromosomes in an embryo. Humans normally have 46 chromosomes arranged in 23 pairs. An embryo with too many or too few has aneuploidy. An extra copy of chromosome 21, for example, causes Down syndrome.

Aneuploidy can lead to failed implantation or miscarriage. PGT-A identifies embryos with the expected number of chromosomes, with the aim of improving the chance of a healthy pregnancy. It is commonly discussed with patients who have had recurrent pregnancy loss, repeated unsuccessful IVF cycles, or who are of older maternal age.

PGT-A is also the most debated of the three, and what the evidence supports is set out under limitations below. It is worth reading that section before deciding.

What is PGT-M and who is it for?

PGT-M, preimplantation genetic testing for monogenic disorders, looks for a specific condition caused by a mutation in a single gene. Where both parents carry a variant for a condition such as cystic fibrosis or sickle cell anemia, there is a risk of passing it to a child. PGT-M screens embryos for that specific condition so unaffected embryos can be identified.

It applies to couples with a known genetic condition in the family, or who are known carriers. Unlike PGT-A, it is testing for something already identified, which is why it is a targeted test rather than a screen.

What is PGT-SR and who is it for?

PGT-SR, preimplantation genetic testing for structural rearrangements, looks at the structure of the chromosomes rather than their number. A translocation, where a piece of one chromosome breaks off and attaches to another, can produce embryos with missing or extra genetic material even though the parent carrying it is healthy.

It applies where one partner is known to carry a chromosomal rearrangement, which is usually discovered after investigation for repeated miscarriage.

How is the testing carried out?

During IVF, eggs are fertilized in the laboratory and the resulting embryos are grown to the blastocyst stage, usually day five or six. A small sample of cells is taken from each embryo and sent to a genetics laboratory. The results tell the specialist which embryos carry the finding being tested for, and embryo transfer is planned accordingly.

What are the limitations of PGT?

No form of PGT guarantees a pregnancy, and none rules out every health condition. Each test answers one specific question. Results are not always clear-cut, and some embryos return an inconclusive result. Testing also adds a step and a cost to the IVF cycle.

For PGT-A specifically, the Human Fertilisation and Embryology Authority rates it red for improving the chance of having a baby for most patients, meaning there is no evidence from randomized controlled trials that it is effective for that purpose, and states that using PGT-A "may decrease the chance of having a baby" because it is a selection tool that reduces the number of embryos available for transfer. The HFEA rates it green for reducing the chance of miscarriage in most patients, and grey in older women, meaning there is not enough good-quality evidence either way in that group.

Those two things can both be true: fewer miscarriages does not automatically mean more babies. This is worth discussing directly with your specialist rather than treating PGT-A as a straightforward upgrade to an IVF cycle.

How do you decide which test applies to you?

It follows from what is already known about your history rather than from preference. PGT-M and PGT-SR apply where a specific genetic or chromosomal finding has already been identified in one or both partners. PGT-A is the one that involves a judgment call, and the evidence above is central to it.

We at Millennium IVF Clinic will go through which, if any, of these tests applies to your situation. You can arrange a consultation with our team.

Frequently asked questions

Does the biopsy harm the embryo?

The biopsy is taken at the blastocyst stage from the cells that would go on to form the placenta rather than the baby. Discuss the specific risks of the procedure with your specialist before deciding.

Can PGT test for everything?

No. Each test looks for one type of finding. PGT-A counts chromosomes and does not detect single-gene conditions; PGT-M looks for one named condition and nothing else.

Does PGT-A improve my chance of having a baby?

The HFEA rates PGT-A red on that specific question for most patients, meaning randomized trial evidence does not show a benefit, and notes it may reduce the chance. It rates it green for reducing miscarriage. Ask your specialist how that applies to your circumstances.