Preimplantation genetic testing (PGT) is an umbrella term for a family of tests that analyse the DNA of IVF embryos before they are transferred to the uterus. The goal is to identify embryos that are chromosomally normal, or free from a specific genetic condition, before transfer — rather than learning about chromosomal problems after implantation or during pregnancy.
The three main types of PGT serve different clinical purposes, and understanding which type applies to your situation is important before engaging with clinic recommendations about testing.
How PGT Works
All forms of PGT follow the same procedural steps:
- IVF is performed as standard, and embryos are cultured to the blastocyst stage (Day 5–6)
- A small number of cells (typically 5–8) are removed from the outer cell layer (trophectoderm) of each blastocyst in a procedure called a biopsy
- The biopsied cells are sent to a specialist genetics laboratory for analysis
- The embryos are frozen while results are awaited (typically 2–4 weeks)
- Embryos that pass the test are transferred in a subsequent frozen embryo transfer (FET) cycle
The biopsy and freezing are well-established procedures. The embryo's capacity to develop is not significantly impaired by removing cells from the trophectoderm (which becomes the placenta, not the embryo itself).
PGT-A: Testing for Chromosomal Abnormalities (Aneuploidy)
What it does: PGT-A screens embryos for abnormal chromosome number — too many or too few chromosomes in any of the 23 pairs. An embryo with the correct chromosome number is called "euploid." An embryo with an incorrect chromosome number is "aneuploid."
Why aneuploidy matters: Aneuploid embryos are the primary reason IVF cycles fail — either through failure to implant, or through miscarriage. Aneuploidy rates rise sharply with the egg provider's age: approximately 30–40% of embryos from a 35-year-old are aneuploid, rising to 70–80% in the mid-forties.
Who it is for: PGT-A is most often considered for:
- Patients who have experienced recurrent implantation failure or recurrent miscarriage
- Patients over 37 (where aneuploidy rates are higher and cycle failure more likely)
- Patients with multiple embryos who want to prioritise which to transfer
The controversy: The HFEA currently classifies PGT-A as an "add-on with limited evidence" and does not recommend it routinely for all patients. The concern is that some euploid-classified embryos may not implant, and some embryos classified as aneuploid — particularly those classified as "mosaic" (a mixture of normal and abnormal cells) — may develop into healthy babies if transferred.
Mosaicism in embryos is a nuanced area. Mosaic embryos are generally not recommended for first-line transfer but may be considered where no euploid embryos are available.
Cost: PGT-A adds approximately £2,000–£4,000 to an IVF cycle, covering biopsy, laboratory analysis, and the FET cycle.
PGT-M: Testing for Single Gene Disorders
What it does: PGT-M (previously called PGD — preimplantation genetic diagnosis) screens embryos for a specific, known inherited single-gene condition that runs in one or both intended parents.
Who it is for: PGT-M is appropriate for patients who carry — or are at risk of having a child with — a serious genetic condition that can be identified at the DNA level. Common conditions tested for include:
- BRCA1/BRCA2 mutations (high breast/ovarian cancer risk)
- Cystic fibrosis
- Huntington's disease
- Duchenne muscular dystrophy
- Sickle cell disease
- Spinal muscular atrophy (SMA)
- Many other monogenic conditions
How it differs from PGT-A: PGT-M does not screen all chromosomes — it tests for the specific mutation or gene variant that has been identified in the family. The laboratory designs a bespoke test specific to each family's mutation, which takes several weeks before a cycle can begin.
HFEA licensing: PGT-M requires HFEA approval for each specific condition. The HFEA maintains a list of conditions for which PGT-M is licensed. Most serious monogenic conditions have already been approved; if your condition is not on the list, an application can be made.
Cost: PGT-M is significantly more expensive than PGT-A, typically £4,000–£8,000 for the testing component alone, reflecting the bespoke test design. Some NHS funding is available for specific indications through genetics referral pathways — discuss with your clinical genetics team.
PGT-SR: Testing for Structural Rearrangements
What it does: PGT-SR screens embryos for chromosomal structural abnormalities — where pieces of chromosomes have been translocated (moved to a different chromosome), inverted, or otherwise rearranged. One or both parents may carry a balanced chromosomal rearrangement, meaning they are personally unaffected but have a high risk of passing on an unbalanced form to children, leading to miscarriage or a child with chromosomal abnormality.
Who it is for: Carriers of balanced translocations or inversions who are experiencing recurrent pregnancy loss or have a family history of chromosomal rearrangement.
Cost: Similar to PGT-A in range, typically £2,500–£4,500, but the specific test must be designed for the individual's rearrangement.
NHS Funding for PGT
PGT-A: Generally not funded by the NHS, as it is classified as an elective add-on.
PGT-M: NHS funding is available for some indications through clinical genetics referral pathways. Typically requires referral from a clinical genetics consultant and meets criteria related to severity of the condition and risk level. Where NHS funding applies, it covers the genetic test itself; the IVF cycle costs may still need to be met privately unless the couple also qualifies for NHS IVF.
PGT-SR: NHS funding may be available where PGT-SR is medically indicated due to a known chromosomal rearrangement, again via clinical genetics pathways.
Frequently Asked Questions
Q: Does PGT-A guarantee a successful pregnancy?
A: No. PGT-A identifies embryos with the correct chromosome number, but chromosomal normality is one of several factors in implantation success. A euploid embryo still needs to implant in a receptive uterus. PGT-A reduces the chance of a chromosomally abnormal pregnancy but does not eliminate the possibility of failed implantation or other causes of pregnancy loss.
Q: What happens if all my embryos are aneuploid?
A: This can happen, particularly in older patients or those with very few embryos. If all tested embryos are aneuploid, the options are: repeat the IVF cycle hoping for a euploid embryo; consider transferring a mosaic embryo with appropriate counselling; consider donor eggs; or review the situation with a specialist. There is no automatic answer — the right path depends on your specific circumstances and values.
Q: Can PGT-A find all genetic conditions?
A: No. PGT-A screens for chromosomal number abnormalities only — it does not test for single-gene conditions. A euploid embryo can still carry a genetic disease mutation. If there is a specific genetic condition in the family, PGT-M (not PGT-A) is the relevant test.
Q: Do I need PGT if I am under 35?
A: PGT-A is less impactful in patients under 35 with good prognosis, because aneuploidy rates are lower and per-cycle success rates without testing are already reasonable. Most specialists do not recommend PGT-A routinely for patients under 35 with no history of implantation failure or miscarriage. It is more useful where there is specific clinical indication.
Q: How do I know if I need PGT-M?
A: PGT-M is relevant if you or your partner carry a known mutation in a serious inherited condition, or have a family history suggestive of one. A referral to a clinical genetics service for counselling and testing is the starting point. The genetics team will advise whether PGT-M is appropriate and can coordinate with a fertility clinic that offers the service.
This article is for information only and does not constitute medical or genetic advice. HFEA licensing and NHS funding criteria change; verify current information with your clinic and clinical genetics team.