PGT Genetic Testing in IVF: How It Works

Preimplantation Genetic Testing (PGT) is a group of laboratory techniques performed on IVF embryos before transfer to the uterus, as described by ESHRE. By analyzing a small biopsy of cells from the blastocyst, PGT can identify chromosomal abnormalities, single-gene disorders, and structural chromosomal rearrangements - allowing the fertility team to select the embryo with the highest potential for a healthy pregnancy.

What Are the Types of PGT?

PGT-A (Aneuploidy Screening)

PGT-A tests whether each embryo has the correct number of chromosomes (46 in humans - 23 pairs). Embryos with too many or too few chromosomes (aneuploid) are the primary cause of IVF implantation failure, miscarriage, and chromosomal conditions. The rate of aneuploidy increases dramatically with maternal age:

• Age 30: ~30% of embryos are aneuploid
• Age 35: ~40% aneuploid
• Age 38: ~60% aneuploid
• Age 40: ~70-80% aneuploid
• Age 43+: ~90% aneuploid

By selecting embryos identified as euploid, PGT-A aims to reduce the two most common adverse outcomes: failed implantation and miscarriage. Professional bodies remain cautious about how much it delivers overall — it selects among the embryos you already have rather than creating more, so it can shorten the route to an answer without raising the cumulative chance of a live birth (ASRM - Genetic screening of embryos).

PGT-M (Monogenic/Single-Gene Disorders)

PGT-M tests for specific inherited genetic diseases when one or both parents are known carriers (Mayo Clinic). It requires custom probe design for each family - the testing protocol is tailored to the specific mutation being screened. Common indications:

• Cystic fibrosis (CFTR mutations)
• Sickle cell disease
• Thalassemia
• Huntington's disease
• BRCA1/BRCA2 (hereditary breast and ovarian cancer)
• Spinal muscular atrophy (SMA)
• Fragile X syndrome

PGT-SR (Structural Rearrangements)

For patients who carry balanced chromosomal translocations or inversions. These structural rearrangements increase the risk of producing embryos with unbalanced chromosomal content - leading to recurrent miscarriage or offspring with chromosomal abnormalities.

How Do PGT-A, PGT-M, and PGT-SR Compare?

All three tests use the same biopsy and the same sequencing workflow, but they answer very different questions. They are not alternatives to one another: a couple who carry a single-gene condition may have both PGT-M and PGT-A reported from one biopsy. The table below sets out where they differ.

The practical difference is planning time. PGT-A can usually be added at short notice, whereas PGT-M and PGT-SR need preparatory laboratory work before stimulation starts. If you are traveling for treatment, that groundwork is worth starting well before any flights are booked.

How Does the PGT Process Work?

1. Standard IVF cycle: Ovarian stimulation, egg retrieval, fertilization, and embryo culture proceed as in a regular IVF cycle.
2. Blastocyst biopsy (Day 5-6): When the embryo reaches the blastocyst stage, an embryologist removes 5-8 cells from the trophectoderm (the outer cell layer that will become the placenta). The inner cell mass (which becomes the baby) is not touched.
3. Vitrification: The biopsied embryo is immediately vitrified (frozen) while awaiting test results.
4. Genetic analysis: The biopsy sample is analyzed using Next-Generation Sequencing (NGS) - the most advanced and accurate platform currently available. Results take 1-2 weeks.
5. Results counseling: The fertility team reviews results with you, categorizing each embryo as euploid, aneuploid, or mosaic.
6. Frozen embryo transfer (FET): A frozen embryo transfer is scheduled for the best euploid embryo in a subsequent cycle.

What to Expect at Each Phase of a PGT Cycle

The steps above describe what the laboratory does. What patients more often want to know is how the cycle feels from the outside, and where the decision points fall.

Phase One: Counseling and Planning

Before medication starts, you meet the fertility team and, for PGT-M and PGT-SR, a genetic counselor. This establishes what is being tested for, what the possible results are, and - importantly - what you would do with each of them. Deciding in advance how you would approach an inconclusive result is far easier than deciding in the moment.

Phase Two: Stimulation and Retrieval

Identical to a standard cycle - PGT changes nothing about the injections, the scans, or the retrieval. The difference is emphasis: because testing sorts through the embryos you already have, the size of that pool matters more than usual. Not every egg fertilizes, and not every fertilized egg reaches the stage at which biopsy is possible.

Phase Three: Biopsy and the Wait

Only embryos reaching the blastocyst stage can be biopsied. Those embryos are vitrified while the sample is analyzed, which is why PGT cycles almost always end in a frozen rather than a fresh transfer. Many patients describe this wait as the hardest part - there is nothing to control, and the call at the end of it may change the plan entirely.

Phase Four: Results and the Transfer Decision

Each embryo is categorized, and the team explains what those categories mean for your situation. Some cycles produce several euploid embryos and the only question is which to transfer first. Others produce none - painful, but informative: it reframes the conversation from "why did the transfer fail?" to "what would give us a better pool next time?"

What the UK Regulator Says About PGT-A

This has to come before anything else on the page, because it is the single most important fact for a UK patient and it is the one clinics selling the test tend not to lead with.

The HFEA rates PGT-A separately for separate outcomes, and the two ratings point in opposite directions. That is the most useful thing on this page, so it is worth getting exactly right.

• Rated red for improving the chance of having a baby for most patients. The regulator's stated reason is specific rather than vague: PGT-A is a selection tool that often reduces the number of embryos available for transfer, and the time to a conception ending in live birth may be longer.
• Rated green for reducing the chance of miscarriage for most patients - on balance, high quality evidence shows it is effective at that. Green is the top of the scale.
• Rated grey for reducing miscarriage in older women, meaning the evidence is insufficient to rate it for that group specifically.

Read together, that says something a sales conversation is unlikely to: PGT-A can genuinely lower your chance of miscarrying, and can at the same time lower your chance of ending up with a baby, because embryos get excluded along the way. The HFEA states both. It also notes that reducing miscarriage does not remove the risk entirely, since aneuploidy is not the only cause. Whether that trade is worth making depends on what you are trying to avoid and how many embryos you have - which is a conversation, not a package inclusion. The regulator's own assessment is on the HFEA's PGT-A page, and we would rather you read it than take our word for it.

This applies to PGT-A only. PGT-M and PGT-SR are a different matter entirely: for a couple who carry a known single-gene condition or a balanced translocation, testing is established practice with a clear clinical purpose, and nothing above is an argument against it. The contested test is the one that screens every embryo for chromosome number in patients who have no known genetic condition.

Why the Per-Transfer Number Is Misleading

Selecting euploid embryos raises the implantation rate per transfer. It would be surprising if it did not - you have removed from the pool the embryos most likely to fail, so the ones left behind perform better. What that number does not tell you is whether you are more likely to take a baby home from the cycle you paid for, and the measure that answers that question is the cumulative live birth rate per cycle started. On that measure the evidence does not support PGT-A for most patients, which is what the red rating reflects.

It matters most when you have few embryos. Test four embryos, have three reported abnormal, and you may transfer nothing at all - having paid extra to arrive at that outcome.

The Risk Nobody Puts on a Price List

• False abnormal results. The biopsy samples a handful of cells from the outer layer that becomes the placenta, not the cells that become the baby, and those cells do not always represent the embryo. An embryo that could have become a healthy child can be reported abnormal and discarded. This is not theoretical, and it is irreversible.
• False normal results. A euploid result is not a guarantee of a healthy baby or of a chromosomally normal pregnancy. Routine antenatal screening is still needed afterwards.
• No result at all. Some biopsies fail to produce a readable result, leaving you with the cost and no information, or a decision about re-biopsying an embryo.
• The biopsy itself. Large-scale use is broadly reassuring on short-term outcomes, but removing cells from a five-day embryo is an invasive procedure and the HFEA's red rating explicitly includes potential safety concerns. "Millions performed worldwide" is a statement about how common it is, not about whether it is harmless.

If You Are Travelling for This

A fertility clinic in Turkey is not HFEA-licensed. That means no HFEA inspection, no HFEA complaints route, and none of the HFEA's rules on embryo transfer limits, consent or storage. It also means the UK restriction on which conditions PGT-M may be used for does not apply: in the UK the HFEA approves conditions individually, and a clinic outside that system is not bound by that list. If you are offered testing for something, ask whether it would be permitted in the UK and why the answer is what it is.

Ask any clinic recommending PGT-A three questions before agreeing: what it costs on top of the cycle, what happens to your cycle if every embryo is reported abnormal, and what evidence they are relying on given the HFEA's rating. The quality of the third answer tells you the most.

Should Mosaic Embryos Be Transferred?

Mosaic embryos contain a mixture of normal and abnormal cells. They represent approximately 10-20% of tested embryos and create a clinical dilemma: should they be transferred?

Current evidence: low-level mosaicism (less than 40% abnormal cells) can result in healthy pregnancies, as the embryo may self-correct during development. High-level mosaicism carries higher risk. PGDIS (Preimplantation Genetic Diagnosis International Society) guidelines recommend discussing mosaic embryo transfer as an option when no euploid embryos are available - with appropriate genetic counseling.

Who Benefits Most from PGT?

• Women over 35: the baseline aneuploidy rate is higher, which is why PGT-A is most often discussed in this group - though the regulator's red rating applies here too, and having fewer embryos increases the cost of a wrong call.
• Recurrent pregnancy loss: 2+ miscarriages. Chromosomal abnormality in the embryo is the single commonest cause of early miscarriage, which is the reasoning behind testing here - and this is the one group where the regulator is positive: the HFEA rates PGT-A green for reducing the chance of miscarriage (HFEA). It also notes that aneuploidy is not the only cause of miscarriage, so this reduces the risk rather than removing it.
• Repeated implantation failure: 3+ failed transfers - selecting euploid embryos may help, though it may also simply confirm that the embryos available are not the problem.
• Known genetic carriers: PGT-M is the only way to prevent transmission of known single-gene disorders without sacrificing the ability to have genetically related children.
• Choosing between several embryos: where a cycle has produced a number of embryos, PGT-A is one way of deciding the order to transfer them - a sequencing tool rather than a way of producing a better outcome.

What PGT Can and Cannot Tell You

The Limits of the Test

• It does not improve any embryo. PGT sorts the embryos a cycle has already produced. It cannot make a poor-quality embryo better, nor increase the number available.
• It is not a general health screen. PGT-A looks at chromosome number; PGT-M looks only for the mutation it was designed to find. No form of PGT screens for every genetic condition.
• The biopsy is a sample, not a census. A few cells from the outer layer are highly informative, but they remain an inference about the rest of the embryo - which is why mosaic results exist and can be ambiguous. Occasionally a sample gives no readable result; that does not mean the embryo is abnormal.
• It cannot predict implantation. Chromosomal normality is one requirement among several. Endometrial receptivity, uterine anatomy, and factors still not fully understood also matter, so a euploid transfer can still fail.
• It does not replace testing during pregnancy. Standard prenatal screening remains appropriate afterwards; your obstetric team will advise.

Common Misconceptions

• "PGT guarantees a healthy baby." It reduces specific, identifiable risks; it does not remove risk in general.
• "PGT means choosing your baby's traits." It reports on chromosome counts, defined mutations, and structural rearrangements. It is a diagnostic test, not a menu.
• "Everyone having IVF should add PGT." Not necessarily. Where few embryos are available, testing may add cost and delay without changing which one is transferred. Whether it helps depends on your history and priorities - discuss it with your fertility specialist rather than assuming it.
• "A mosaic embryo is a failed embryo." Not automatically, as above. Mosaic results warrant counseling, not a reflex discard.

Where the Rules Differ

PGT sits inside a regulatory framework as well as a clinical one, and that framework is not the same everywhere. What PGT may be used for, which indications are accepted, and how long untransferred embryos may be stored are governed by national rules that differ between countries and change over time. Do not assume the position you know from home applies at your destination. Verify the specifics with the clinic and the relevant national regulator before booking, and ask for the answer in writing.

Questions to Ask Your Fertility Specialist

For most patients PGT is an elective addition, which makes it a decision rather than a given:

• Given my age, history, and ovarian reserve, what would PGT realistically change about my plan?
• How many of my embryos would you expect to be testable, and is that enough for testing to be worthwhile?
• Which type of PGT are you recommending, and why that one?
• What happens - clinically and financially - if no embryo is suitable for transfer?
• How does your laboratory report mosaic results, and what is your policy on transferring them?
• Is genetic counseling included before and after testing?
• What do the rules here permit PGT to test for, and can you confirm that in writing?

Where partner fertility centres offer PGT it is performed using next-generation sequencing (NGS), which published studies report to be accurate for detecting chromosomal abnormalities - accurate at reading the cells sampled, which is a different question from whether those cells represent the embryo. Which of the three PGT types a given centre offers varies, so confirm it before you book. Transparent IVF pricing includes PGT costs in comprehensive packages, with results typically available within 10-14 days. Genetic counseling is provided before and after testing to ensure patients fully understand results and implications.