What Is PGT-A? How Embryo Genetic Testing Works During IVF

Your doctor has mentioned PGT-A. Or you've come across the term while researching IVF and want to understand what it actually involves before your next consultation. Either way, this guide explains exactly what PGT-A is, how the process works step by step, who it is most likely to benefit, and what the results mean for your treatment.

PGT-A does not replace the foundational fertility investigations that should come before any IVF cycle. But for the right patients, it is one of the most clinically significant advances in embryo selection available today.

PGT-A stands for Preimplantation Genetic Testing for Aneuploidy. It is a laboratory procedure performed on embryos created through IVF, before any embryo is transferred to the uterus. The purpose is to identify which embryos have the correct number of chromosomes — 46 in total, arranged in 23 pairs — and which carry chromosomal abnormalities (aneuploidy) that would prevent a successful pregnancy.

Aneuploidy is one of the leading causes of IVF failure, implantation failure, and early miscarriage. It becomes more common as women age — by their early 40s, the majority of a woman's eggs may carry chromosomal errors. PGT-A allows embryologists to identify chromosomally normal embryos before transfer, rather than discovering problems only after a failed cycle or pregnancy loss.

The test analyses all 24 chromosome types (22 autosomes plus the X and Y sex chromosomes) simultaneously, giving a complete picture of each embryo's chromosomal status.

PGT-A is integrated into the standard IVF process. Here is what happens at each stage:

PGT-A is one of three types of preimplantation genetic testing. They are often confused but serve different purposes:

Most couples encounter PGT-A first because it screens for the most common cause of IVF failure — chromosomal number errors — without requiring prior knowledge of a specific genetic condition. PGT-M and PGT-SR are used when a known genetic issue has already been identified.

PGT-A is not recommended for every IVF patient. The clinical evidence is strongest for specific groups where the rate of chromosomal abnormality is high enough to make embryo selection meaningful.

PGT-A results classify embryos into three categories — and the third is the most complex:

• Euploid — chromosomally normal. Recommended for transfer.
• Aneuploid — chromosomally abnormal. Not recommended for transfer as they are unlikely to result in a viable pregnancy and carry a high miscarriage risk.
• Mosaic — a mixture of normal and abnormal cells within the same embryo. This is where clinical decision-making becomes nuanced.

Mosaic embryos were not well understood in earlier generations of genetic testing. Improved sequencing technology now detects them more reliably — and the evidence suggests that some mosaic embryos can implant and result in healthy pregnancies, while others carry significant risk.

The degree of mosaicism matters. A low-level mosaic embryo (where the proportion of abnormal cells is small) may be considered for transfer when no euploid embryos are available, following detailed counselling. High-level mosaic embryos carry substantially higher miscarriage risk and are usually not transferred.

The decision to transfer a mosaic embryo is always made on a case-by-case basis with your clinical and genetics team — it is not a straightforward yes or no.

PGT-A is a sophisticated procedure but it carries real limitations that every patient should understand before deciding whether to proceed.

One limitation that patients are often surprised by: if all embryos in a cohort are aneuploid, PGT-A leaves no embryos available for transfer. This can be emotionally difficult — particularly for older patients who produced few eggs. Understanding this possibility before starting a PGT-A cycle is important for managing expectations.

PGT-A is not universally recommended, and there is genuine scientific debate about its benefit for certain patient groups. This is worth understanding before deciding.

Where the evidence is strong: For women over 35, those with recurrent implantation failure, and those with recurrent miscarriage, multiple randomised controlled trials show that PGT-A improves the live birth rate per transfer and reduces miscarriage rates. The benefit is clearest because these patients have the highest background rate of chromosomal abnormality.

Where the evidence is less clear: For women under 35 with no history of failure, the picture is more complicated. The baseline rate of aneuploidy is lower in this group, meaning PGT-A may identify fewer abnormal embryos while still carrying the risks of biopsy. Some studies suggest that in good-prognosis younger patients, PGT-A does not meaningfully improve cumulative live birth rates — and may even reduce them slightly, by discarding embryos that were classified as aneuploid but might have self-corrected.

The mosaic embryo question: As detection of mosaic embryos has improved, so has awareness that mosaic classification is not always definitive. The biopsy samples only a small number of cells from the trophectoderm — it may not fully represent the chromosomal composition of the entire embryo. This is known as sampling error, and it is an ongoing area of research.

The consensus position from the HFEA and most UK fertility specialists is that PGT-A is a beneficial add-on for specific patient groups — but should not be presented as a routine improvement for all IVF patients. Ask your clinical team which category you fall into.

Results are typically returned within 14 to 19 days of the biopsy. Your clinic will receive a report classifying each embryo as euploid, aneuploid, or mosaic. Here is what happens next depending on the outcome:

For patients with euploid embryos, the frozen embryo transfer cycle typically begins in the following weeks. The endometrium is prepared with medication, and the embryo is transferred in a straightforward procedure similar to a standard FET.

Having a euploid embryo does not guarantee a successful pregnancy — other factors including uterine receptivity, endometrial thickness, and underlying health conditions also influence implantation. But it does significantly improve the probability per transfer compared to transferring an untested embryo.