# PGT-A : testing embryos for chromosome number > What PGT-A (preimplantation genetic testing for aneuploidy) is, how embryos are tested, what the evidence shows, and its risks and limits, including mosaicism. Source: https://www.ghinashami.com/en/interests/pgt-a/ Updated: 2026-10-13 Step 3 of 4 · Assisted reproduction PGT-A (preimplantation genetic testing for aneuploidy) checks embryos created through IVF for an abnormal number of chromosomes before one is chosen for transfer. It is widely used, but its benefit is debated. This page explains what it can and cannot do. - 4 min read - Reviewed 13 October 2026 - 6 references ## What PGT-A is Embryos with a missing or extra chromosome (aneuploidy) are less likely to implant and more likely to miscarry, and aneuploidy becomes more common as the woman's age increases. PGT-A, previously called preimplantation genetic screening (PGS), counts the chromosomes in a few cells taken from each embryo, with the aim of choosing an embryo with the expected number for transfer. PGT-A is different from [PGT-M](https://www.ghinashami.com/en/interests/pgt-m/), which looks for one specific inherited condition already known in the family. ## How it is done PGT-A requires an [IVF or ICSI](https://www.ghinashami.com/en/interests/ivf-icsi/) cycle. Embryos are usually grown to the blastocyst stage, and an embryologist removes a few cells from the trophectoderm, the outer layer that goes on to form the placenta rather than the baby. The embryos are frozen while the cells are analysed, and an embryo is transferred in a later frozen cycle. ## What the evidence shows Randomised trials have not shown that PGT-A increases the chance of having a baby for most people having IVF. Guideline bodies have reached similar conclusions: - **NICE (2026)** advises against offering PGT-A as part of fertility treatment to improve live birth rates, noting that it can reduce the number of embryos available for transfer. - **The HFEA** rates PGT-A red for improving the chance of having a baby for most fertility patients, a rating it uses when there are possible concerns about harm or when moderate to high-quality evidence suggests an add-on may reduce treatment effectiveness. It rates PGT-A green for reducing the chance of miscarriage for most patients, and grey (not enough good evidence to rate) for older women. - **ESHRE (2023)** does not recommend PGT-A for routine clinical use. - **The ASRM (2024)** concludes that the value of PGT-A as a routine test for all IVF patients has not been demonstrated and that its value in lowering the risk of miscarriage is unclear; for couples with recurrent pregnancy loss, it finds definitive evidence of benefit lacking. ## Questions worth asking your doctor 1. 1 What would PGT-A add in our situation? Ask what the evidence shows for people of your age and history, and how the findings would change the plan. 2. 2 How are mosaic embryos handled? Policies on transferring and freezing mosaic embryos differ between centres, and ESHRE advises that they be explained during genetic counselling. 3. 3 What if no embryo is suitable for transfer? PGT-A can leave fewer embryos, or none, to transfer in a cycle; ask how that situation would be managed. Dr. Ghina Shami ## Risks and limits **An IVF cycle is needed.** PGT-A carries the risks of IVF, including ovarian hyperstimulation and multiple pregnancy if more than one embryo is transferred; see [IVF and ICSI](https://www.ghinashami.com/en/interests/ivf-icsi/). **Harm to the embryo.** Removing cells may damage an embryo and stop it developing. **Wrong or uncertain findings.** The test can miss an abnormality or report one that is not there. Some embryos are mosaic: they contain cells with different chromosome numbers, so a few cells from the outer layer may not represent the whole embryo. Mosaic embryos may have a lower chance of pregnancy, but healthy babies have been born after their transfer. **Fewer embryos.** Embryos that could have become a healthy baby may be discarded, which can leave fewer embryos, or none, to transfer. **Prenatal testing still matters.** ESHRE advises that counselling about prenatal diagnosis be offered to everyone who becomes pregnant after PGT. Waiting for the report and making decisions about embryos can also be emotionally demanding. ## Common questions **Does PGT-A increase the chance of having a baby?** For most people having IVF, randomised trials have not shown a higher live birth rate with PGT-A, and NICE advises against using it for that purpose. The HFEA rates it green for reducing the chance of miscarriage for most patients; the ASRM considers its effect on miscarriage unclear. **How accurate is PGT-A?** It is not completely accurate. It can miss an abnormality or report one that is not there, and mosaicism makes some findings uncertain, which is why embryos that could have developed normally are sometimes discarded. **Is PGT-A the same as PGT-M?** No. [PGT-M](https://www.ghinashami.com/en/interests/pgt-m/) looks for one specific inherited condition known in the family. PGT-A counts the chromosomes in each embryo and does not look for particular inherited conditions. **Does PGT-A require IVF even without a fertility problem?** Yes. Embryos can be tested only if they are created in the laboratory, so a full IVF cycle with stimulation, egg collection and embryo culture is needed. **Evidence basis:** This page is based on guidelines and expert opinions from NICE, ESHRE and the ASRM and on the HFEA's evidence ratings; the benefit of PGT-A is still debated, and the page sets out where the evidence is uncertain. ## References 1. Pre-implantation genetic testing for aneuploidy (PGT-A), HFEA (2023) — [hfea.gov.uk](https://www.hfea.gov.uk/treatments/treatment-add-ons/pre-implantation-genetic-testing-for-aneuploidy-pgt-a/) 2. Treatment add-ons with limited evidence, HFEA (2026) — [hfea.gov.uk](https://www.hfea.gov.uk/treatments/treatment-add-ons/) 3. Fertility problems: assessment and treatment, NICE (guideline NG257) (2026) — [nice.org.uk](https://www.nice.org.uk/guidance/ng257) 4. Good practice recommendations on add-ons in reproductive medicine, ESHRE, Human Reproduction (2023) — [doi:10.1093/humrep/dead184](https://doi.org/10.1093/humrep/dead184) 5. The use of preimplantation genetic testing for aneuploidy: a committee opinion (2024), American Society for Reproductive Medicine (2024) — [asrm.org](https://www.asrm.org/practice-guidance/practice-committee-documents/the-use-of-preimplantation-genetic-testing-for-aneuploidy-a-committee-opinion-2024) 6. ESHRE survey results and good practice recommendations on managing chromosomal mosaicism, ESHRE, Human Reproduction Open (2022) — [doi:10.1093/hropen/hoac044](https://doi.org/10.1093/hropen/hoac044) Written and reviewed by [Dr. Ghina Shami](https://www.ghinashami.com/en/about/), Consultant, Reproductive Medicine and Infertility · 13 October 2026 · [How these pages are prepared](https://www.ghinashami.com/en/legal/#accuracy)