PGT-A: preimplantation genetic testing for aneuploidy

PGT-A, or preimplantation genetic testing for aneuploidy, is a laboratory test carried out on embryos created through IVF. It checks whether each embryo has the expected 46 chromosomes — 23 inherited from the egg and 23 from the sperm — before an embryo is selected for transfer to the womb.

The aim is to identify embryos that are chromosomally balanced (euploid), in the hope of choosing the one most likely to lead to a healthy pregnancy. PGT-A does not change or treat an embryo; it only provides information to help the fertility team and prospective parents decide which embryo to transfer, or whether to transfer at all.

It is important to be clear about who does what. London Pregnancy Clinic does not carry out IVF or the embryo biopsy itself — that happens at a fertility centre. What we provide is clear, unbiased genetic counselling and information around PGT-A, so you can approach the decision with confidence.

Aneuploidy is the medical term for having the wrong number of chromosomes — an extra chromosome or a missing one. Most people have 23 pairs (46 chromosomes in total); an aneuploid embryo has too many or too few.

Aneuploidy is very common in human eggs and embryos, and the chance that an egg carries the wrong number of chromosomes rises as a woman gets older, particularly from the late 30s. Many aneuploid embryos fail to implant or lead to early miscarriage, and chromosomal abnormalities are the single most common cause of first-trimester miscarriage.

Some forms of aneuploidy are compatible with a pregnancy continuing. An extra copy of chromosome 21, for example, causes Down's syndrome. This is why chromosome number matters so much when an embryo is being considered for transfer.

PGT-A is sometimes discussed with people who already need IVF and who fall into groups where aneuploidy may play a larger role. It is not routinely recommended for everyone having IVF. Situations where it is more often raised include:

• Advanced maternal age — the chance of embryo aneuploidy rises with the age of the eggs, especially from the late 30s
• Recurrent miscarriage — two or more pregnancy losses, where chromosomal causes are common
• Repeated IVF failure — several embryo transfers that have not led to a pregnancy
• A previous pregnancy affected by a chromosomal condition
• Couples with several embryos who wish to reduce the number of transfers by prioritising euploid embryos

PGT-A is easily confused with other genetic tests, and the differences matter. PGT-A only counts chromosomes — it does not look for inherited single-gene conditions.

It is different from carrier screening, which checks whether you and your partner both carry a change in the same gene that could be passed to a child. Where a specific inherited condition is a concern, embryos can instead be tested with the sister test, PGT-M (preimplantation genetic testing for monogenic disorders). Our team can help you understand which, if any, is relevant to your circumstances.

PGT-A is only possible as part of an IVF cycle, because it needs embryos created in the laboratory. The biopsy and testing are carried out at the IVF or fertility centre and its genetics laboratory — not at London Pregnancy Clinic. In broad terms, the pathway follows the usual IVF steps with an added testing stage:

• Ovarian stimulation and egg collection, followed by fertilisation in the laboratory
• Embryos are grown for around five to six days to the blastocyst stage
• A few cells are gently removed (biopsied) from the outer layer that would go on to form the placenta
• The embryos are frozen while the cells are analysed for chromosome number
• Chromosomally balanced (euploid) embryos can then be transferred in a later frozen cycle

For some people, PGT-A may reduce the time to pregnancy by helping to avoid transferring embryos that would not implant or would miscarry, and it can lower the chance of a miscarriage caused by aneuploidy. For those with many embryos to choose from, it may reduce the number of transfer cycles.

However, PGT-A is classed by the Human Fertilisation and Embryology Authority (HFEA), the UK fertility regulator, as a treatment 'add-on'. The HFEA's view is that there is not enough good-quality evidence that PGT-A improves the chances of having a baby for most fertility patients, and that in some situations it may not be helpful. Key limitations to understand include:

This is why PGT-A remains genuinely debated among fertility specialists. A result is a probability, not a guarantee, and PGT-A does not replace the prenatal screening you would still be offered in pregnancy, such as NIPT and the routine scans. If a transfer is successful, we can support those early weeks with dedicated IVF pregnancy monitoring.

• PGT-A cannot create good embryos — it can only assess the ones you have, and testing may leave fewer embryos available to transfer
• Some embryos are 'mosaic' (a mix of normal and abnormal cells), which is difficult to interpret and may lead to a potentially viable embryo being set aside
• It does not test for single-gene conditions and cannot guarantee a healthy baby
• The biopsy carries a small risk to the embryo

Whether or not to have PGT-A is a personal decision that benefits from clear, unbiased information rather than sales pressure. London Pregnancy Clinic provides genetic counselling with specialists who can explain what PGT-A can and cannot tell you, and help you balance the potential benefits against the costs, risks and emotional impact.

Because we are an independent early-pregnancy and genetics clinic rather than an IVF provider, our counselling is not tied to selling you a cycle of treatment. We can talk through your history, discuss related options such as carrier screening, and coordinate with your chosen fertility centre so that any decision you make is fully informed.

This page is for general information and education and does not constitute medical advice; please speak to a qualified healthcare professional about your own circumstances. If you have concerns about your pregnancy or your baby's wellbeing, contact your midwife, GP or maternity unit promptly.