What Is PGT? Chromosomal and Genetic-Disease Screening Explained

The three types at a glance

Older terms are still common. PGT-M used to be called PGD (preimplantation genetic diagnosis), and PGT-A was often called PGS (screening).

PGT-M: testing for a known inherited condition

PGT-M is used when a family already knows which gene change it is trying to avoid passing on. The UK regulator says it may be considered for people who have ended a pregnancy because of a serious genetic condition, already have a child with one, or have a family history of one (HFEA, reviewed July 2026).

In the UK, PGT-M can be used for almost any condition linked to a known gene, but only once the HFEA has judged that the condition meets legal criteria on how likely it is to be passed on and how serious it is. The HFEA states that over 2,000 genetic conditions have been approved for PGT-M testing. Approving a new condition is a lengthy process with no guaranteed outcome. Other countries regulate this differently, so if you are having treatment abroad, ask what rules apply there.

Because the test is designed around a specific family variant, there is usually preparation work before IVF begins. In the UK, the HFEA describes the first step as seeing a GP for referral to a regional clinical genetics service.

Why ICSI is often used with PGT-M

According to ASRM's 2026 guidance on ICSI, ICSI may be appropriate for PGT-M because it can reduce contamination from extra sperm and surrounding cells, which could affect the accuracy of the test. By contrast, it says ICSI is not routinely needed for PGT-A without a male-factor reason.

PGT-SR: testing for a chromosome rearrangement

Some people carry a balanced chromosome rearrangement. All their genetic material is present, but pieces are in unusual places. They are usually healthy, but their embryos can end up with missing or extra pieces, which can lead to miscarriage or failed implantation. The HFEA describes PGT-SR as looking at embryo chromosome structure for deleted, duplicated or inverted segments where a known rearrangement is present. It notes that specific approval may not be needed in the UK when the rearrangement is known to increase miscarriage risk.

For people whose rearrangement was found after repeated pregnancy loss, our article on recurrent miscarriage and PGT looks at that situation in more depth.

PGT-A: counting chromosomes

PGT-A checks whether an embryo has the expected number of chromosomes. The ASRM Practice Committee (2024) describes it as analysing all 24 chromosomes, usually from a biopsy of the trophectoderm (the outer cells of a blastocyst), so that only euploid embryos are transferred. ASRM notes that its use in US IVF cycles rose from 14% in 2014 to 44% in 2019, according to SART data.

The evidence on whether it helps is mixed. ASRM's key conclusions:

• Routine PGT-A for all IVF patients "cannot be recommended," based on randomised trial evidence.
• In a 2019 multicentre trial of 661 patients aged 25 to 40, live birth rates did not differ significantly, either per transfer or per patient who started treatment.
• It is not recommended routinely for male-factor infertility alone, or for older paternal age.
• For older maternal age, a discussion of PGT-A may be reasonable, especially with good ovarian reserve, though the evidence is limited.

Our article on PGT-A for advanced maternal age goes through that group's evidence in detail.

What every type of PGT involves

All three follow the same basic path:

1. IVF. Embryos have to be created in the lab, so PGT always requires a full IVF cycle.
2. Biopsy. The lab removes a small sample of cells from each embryo, usually once it has reached the blastocyst stage.
3. Freezing and waiting. The HFEA notes that transfer is now often of frozen embryos while results are processed.
4. Results. Each embryo is reported as unaffected, affected, or with another result such as "inconclusive" or "mosaic."
5. Transfer. An embryo with a suitable result is transferred in a later cycle.

Learning to read the report is a skill of its own; our guide to how PGT results are reported walks through the terms.

What PGT cannot do

PGT is a targeted tool, and its limits matter as much as its uses.

• It is not 100% accurate. The HFEA notes that tests may fail or give incorrect results.
• Some biopsies give no answer. ASRM reports inconclusive PGT-A results in about 0.86% to 3.8% of biopsies.
• Biopsy can damage embryos. The HFEA says some embryos may become unusable.
• There may be nothing to transfer. If few eggs fertilise, or every embryo is affected, there may be no suitable embryo, as the HFEA points out.
• It does not check for everything. PGT-M looks for one known condition; PGT-A counts chromosomes. Neither is a general health screen of the embryo.
• Some results are uncertain. PGT-A can report "mosaic" findings, which are hard to interpret. See our explainer on mosaic embryos.

This article deals only with the medical uses of embryo testing described above. It does not cover choosing embryos by sex or by any non-medical trait.

If you are having PGT abroad

PGT adds steps that affect travel plans. Because embryos are usually frozen while results are processed, transfer typically happens in a later cycle, which often means a second trip. Biopsy samples are not always tested at the clinic itself; some are sent to an outside genetics laboratory, possibly in another country. Ask where your samples will go, how long results usually take, and who will explain them to you. For PGT-M, check early whether the destination clinic can design a test for your family's specific variant, and what records from your genetics team it will need.

Counselling and consent

ASRM's 2024 opinion says consent for PGT-A should cover its risks, benefits and limitations, including the possibility of no result, mosaic or segmental results, misdiagnosis, and the option not to test. It adds that a clinic's rules for embryos that test abnormal should be set down in writing and given to patients before any biopsy takes place. Access to genetic counselling is recommended.

Questions to ask before choosing PGT

• Which type of PGT fits my situation, and why?
• For PGT-M or PGT-SR: is preparatory work needed, and how long does it take?
• Which laboratory will do the testing, and how are results reported?
• What happens to embryos with abnormal, inconclusive or mosaic results?
• Will I see a genetic counsellor before and after testing?
• Could I end up with no embryo to transfer, and how likely is that for me?

Frequently asked questions