Preimplantation Genetic Testing (PGT)

Introduction

If you are reading this, you are likely already on an IVF path, or have just been told that Preimplantation Genetic Testing (PGT) may be relevant for your situation. PGT is a laboratory test performed on embryos created through IVF, before any embryo is placed in the uterus. It is used to look for chromosomal differences or specific inherited conditions that could affect implantation, pregnancy, or the future child’s health.

PGT is not a treatment in itself. It does not change an embryo or fix anything. It provides information — information that fertility specialists and families can use to decide which embryo to transfer first, or whether an embryo is suitable for transfer at all.

This guide explains the three main forms of PGT (PGT-A, PGT-M, and PGT-SR), how the test fits into an IVF cycle, what genetic counselling involves, what the results can and cannot tell you, the risks and limitations, and the practical and emotional aspects of going through testing. The aim is to help you have a clearer, more informed conversation with your fertility specialist and genetic counsellor.

What Is Preimplantation Genetic Testing?

Preimplantation Genetic Testing, usually shortened to PGT, is the umbrella term for genetic tests performed on embryos during an IVF cycle. The embryos are created in the laboratory, grown for several days, and a small number of cells are removed from each embryo for analysis. Those cells — not the embryo itself — are sent to a genetics laboratory. The embryos are frozen while the results are awaited.

The test was previously known as preimplantation genetic diagnosis (PGD) and preimplantation genetic screening (PGS). Most professional societies, including the European Society of Human Reproduction and Embryology (ESHRE), now use the broader term PGT, with sub-types defined by what is being looked for.

The key things to understand from the start:

• PGT can only be done as part of an IVF cycle, because embryos are needed in the laboratory.
• PGT provides information about the embryos that exist. It does not improve embryo quality, increase the number of embryos, or treat any condition.
• PGT reduces but does not eliminate uncertainty. A normal PGT result lowers, but does not remove, the risk of miscarriage, implantation failure, or a child being born with a genetic condition.
• PGT is not part of every IVF cycle. Major societies describe it as appropriate in specific clinical situations rather than as routine.

Types of PGT

PGT-A (for Aneuploidy)

PGT-A looks at the number of chromosomes in the embryo. A typical human cell has 46 chromosomes, arranged in 23 pairs. An embryo with the correct number is called euploid. An embryo with a missing or extra chromosome is called aneuploid. Aneuploidy is a common reason for failed implantation, early miscarriage, and certain chromosomal conditions in babies, including Down syndrome (an extra copy of chromosome 21).

The chance of aneuploidy rises sharply with the age of the egg, which is why PGT-A is often discussed for patients in their later reproductive years. PGT-A does not change the underlying age-related egg quality; it simply helps identify, from a given group of embryos, which ones appear chromosomally normal.

Professional opinion on routine PGT-A is mixed. ESHRE and the American Society for Reproductive Medicine (ASRM) have noted that while PGT-A may improve outcomes per embryo transfer in certain groups, it has not been shown to increase the overall chance of a live birth per IVF cycle started in the general population. Whether PGT-A is helpful in a particular case is a clinical decision that depends on age, embryo number, prior history, and goals.

PGT-M (for Monogenic conditions)

PGT-M is used when one or both intended parents are known to carry a specific inherited single-gene condition, and there is a meaningful chance of passing it to a child. Examples include:

• Beta thalassaemia and sickle cell disease
• Cystic fibrosis
• Spinal muscular atrophy
• Duchenne muscular dystrophy
• Huntington’s disease
• Certain inherited cancer syndromes, such as BRCA1 and BRCA2 related cancers
• Other single-gene disorders identified through carrier screening or family history

PGT-M usually requires a custom test designed for the specific family and the specific mutation. This design phase can take several weeks and may need blood samples from the couple and, in some cases, other family members. PGT-M is the form of PGT most clearly supported by major societies, because the clinical question — whether an embryo carries a specific known mutation — can be answered with a high degree of certainty.

PGT-SR (for Structural Rearrangements)

PGT-SR is used when one parent is known to carry a structural rearrangement of the chromosomes, such as a balanced translocation or inversion. The parent is usually healthy, because all the genetic material is present — it is just arranged differently. However, the eggs or sperm they produce can carry unbalanced versions of the rearrangement, which often leads to miscarriage or, less often, a child with a chromosomal disorder.

PGT-SR identifies embryos that have inherited a balanced or normal chromosome arrangement, which are the ones suitable for transfer. It is most often considered after a karyotype test (a blood test that examines chromosomes) has identified a translocation, frequently following recurrent miscarriage or an affected pregnancy.

Who Is PGT For?

PGT is not part of every IVF cycle, and major societies are clear that it should be discussed in specific clinical contexts rather than offered as a default. Fertility specialists commonly raise PGT in the following situations.

Situations where PGT-A may be discussed

• Advanced maternal age, where the chance of aneuploid embryos is higher
• Recurrent miscarriage, especially where chromosomal causes have been suspected
• Repeated implantation failure in IVF cycles where embryo quality looked good on appearance
• A previous pregnancy with a chromosomal abnormality
• Severe male factor infertility, in some situations

Situations where PGT-M is typically considered

• One or both parents are known carriers of a single-gene condition, identified through family history or carrier screening
• A previous child has been affected by an inherited condition
• One parent has, or is at risk of, an adult-onset inherited disease such as Huntington’s disease
• One parent carries a known inherited cancer-predisposition gene

Situations where PGT-SR is typically considered

• A balanced translocation or other structural rearrangement has been identified in one parent
• Recurrent miscarriages have been linked to a chromosomal rearrangement

For couples without these specific concerns, PGT may not add value, and the cost in time, embryos, and emotional energy may not be justified. Whether PGT is appropriate in a particular case is a clinical decision made with the fertility specialist and, where relevant, a genetic counsellor.

Genetic Counselling Before PGT

Genetic counselling is a core part of the PGT process, particularly for PGT-M and PGT-SR. A genetic counsellor or clinical geneticist helps couples understand:

• The condition being tested for, how it is inherited, and what it means for the family
• The chance that any given embryo would be affected, unaffected but a carrier, or completely unaffected
• The accuracy and limitations of the proposed PGT test
• What the possible results may look like, including the possibility of no suitable embryos
• Alternatives to PGT, including prenatal diagnosis during pregnancy, use of donor gametes (eggs or sperm), adoption, or choosing not to test
• The ethical and emotional aspects of decisions about embryos

Major societies, including ESHRE, describe genetic counselling as an essential step before PGT-M and PGT-SR, and recommend that it is offered for PGT-A as well. Many fertility centres include this as a structured appointment before any IVF medication is started.

The PGT Process: Step by Step

Step 1: Workup and counselling

Before the IVF cycle begins, the fertility team confirms whether PGT is appropriate and which form is needed. Workup typically includes:

• Detailed personal and family medical history
• Carrier screening blood tests for both partners, looking for inherited conditions
• Karyotype (chromosome analysis) where relevant
• Standard fertility tests, including hormonal blood tests and semen analysis
• Genetic counselling, particularly for PGT-M and PGT-SR
• For PGT-M, design of a custom test for the specific family mutation, which can take several weeks

Step 2: Ovarian stimulation and egg retrieval

The IVF cycle proceeds in the usual way. Hormone injections stimulate the ovaries to develop multiple eggs over about ten to fourteen days. The eggs are then collected during a short procedure under sedation, called egg retrieval or oocyte pickup.

Step 3: Fertilisation

The retrieved eggs are fertilised in the laboratory, usually by intracytoplasmic sperm injection (ICSI), where a single sperm is injected into each mature egg. ICSI is generally preferred for cycles that will include PGT, because it reduces the risk of stray sperm DNA contaminating the genetic test.

Step 4: Embryo culture to the blastocyst stage

The fertilised eggs are grown in a specialised incubator for around five to six days. By this point, the most viable embryos reach the blastocyst stage, where the embryo has formed a hollow ball of cells with two distinct parts: the inner cell mass, which will form the future baby, and the trophectoderm, which will form the placenta.

Not all fertilised eggs reach the blastocyst stage. This is normal. PGT can only be performed on embryos that develop this far.

Step 5: Embryo biopsy

On day 5 or 6 of development, the embryologist performs the embryo biopsy. A small opening is made in the outer shell of the embryo (the zona pellucida) using a laser, and a few cells — typically around five to ten — are gently removed from the trophectoderm. The inner cell mass, which will form the baby, is not touched.

Trophectoderm biopsy at the blastocyst stage is considered the standard approach today by ESHRE and ASRM. Earlier methods, which sampled cells from day-3 embryos, are no longer routine because they are now thought to be more disruptive to the embryo.

Step 6: Embryo freezing (vitrification)

After biopsy, the embryos are frozen using a rapid freezing technique called vitrification. They remain in storage while the laboratory analyses the biopsy samples. Vitrification has very high survival rates for blastocysts in experienced laboratories.

Step 7: Genetic analysis

The biopsy samples are sent to a genetics laboratory, where the DNA is amplified and analysed using techniques such as next-generation sequencing (NGS). The laboratory looks for the specific question that PGT-A, PGT-M, or PGT-SR is intended to answer. Results usually take one to several weeks, depending on the test type and the laboratory.

Step 8: Result review and embryo selection

Once the results are available, the fertility team and the patients review them together, often with input from a genetic counsellor. Embryos are typically classified as:

• Suitable for transfer — usually meaning chromosomally normal and unaffected by the specific condition tested
• Not suitable for transfer — affected, aneuploid, or otherwise outside the criteria
• Mosaic — in PGT-A, some embryos show a mixture of normal and abnormal cells in the biopsy sample. The clinical management of mosaic embryos has evolved, and decisions about whether to transfer a mosaic embryo are made on a case-by-case basis with counselling.
• Inconclusive — rarely, the test cannot give a clear answer and a repeat biopsy or alternative approach may be discussed

Step 9: Frozen embryo transfer

A suitable embryo is selected for a frozen embryo transfer (FET) in a later cycle. The uterus is prepared with hormone medications, the embryo is thawed, and a single embryo is usually transferred. Transferring one embryo at a time — rather than two or more — is the approach now favoured by major societies, particularly when PGT has been used to identify a suitable embryo, because it reduces the risks associated with twin or higher-order pregnancies.

Step 10: Pregnancy testing and confirmatory tests

About ten to fourteen days after transfer, a blood test measures pregnancy hormone (beta hCG). If pregnancy is confirmed, routine prenatal care begins. Even when PGT has been used, fertility specialists and obstetricians generally still recommend the usual prenatal screening and, in some situations, confirmatory prenatal diagnostic testing such as chorionic villus sampling (CVS) or amniocentesis, because PGT is not 100% accurate.

What PGT Can and Cannot Tell You

Understanding the limits of PGT is as important as understanding what it can do.

What PGT can do

• Identify embryos with the correct number of chromosomes (PGT-A)
• Identify embryos affected, unaffected, or carrier for a specific known inherited condition (PGT-M)
• Identify embryos with balanced chromosome arrangements when a parent carries a translocation (PGT-SR)
• Help select an embryo for transfer when more than one is available
• Support single embryo transfer, which lowers the risks of twin pregnancies
• Reduce, but not eliminate, the chance of miscarriage from chromosomal causes

What PGT cannot do

• Make an embryo healthier or change its quality
• Guarantee a healthy baby. Many causes of pregnancy loss and birth differences are not detectable by PGT.
• Test for conditions other than those the test is designed to look for. PGT-M for cystic fibrosis, for example, will not tell you about other conditions.
• Replace prenatal screening or diagnostic testing during pregnancy
• Increase the number of embryos available. PGT works with the embryos that develop; it cannot create more.

It is also important to know that some IVF cycles produce no embryos suitable for transfer after PGT. This is a real and difficult outcome, and the chance increases with maternal age. Genetic counselling before starting a cycle should include an honest discussion of this possibility.

Accuracy and Limitations

PGT is highly accurate in experienced laboratories, but it is not perfect. Sources of error and uncertainty include:

• Mosaicism — the biopsy sample may not reflect the whole embryo. An embryo may have a mix of normal and abnormal cells, and the biopsied area may not be representative. This is one reason confirmatory prenatal testing remains relevant.
• Technical errors — though rare, sample contamination, amplification failure, or interpretation issues can occur.
• Coverage limits — PGT-M tests for the specific mutation it was designed for. It does not test for other conditions. PGT-A tests for whole-chromosome differences but does not detect small genetic changes within a chromosome.
• Mosaic results — in PGT-A, embryos with mosaic results present a complex decision. Practices vary, and decisions are individualised with the help of genetic counselling.

Because of these limitations, major societies advise that PGT results be discussed with a genetic counsellor and that standard prenatal screening continue once pregnancy is confirmed.

Success Rates: What Outcomes Can You Expect?

It is natural to want a clear number for what PGT will do for your chances. Honest answers are more useful than confident ones.

Several patterns are well established in clinical research:

• Transferring a euploid (chromosomally normal) embryo, identified by PGT-A, is associated with a higher implantation rate and a lower miscarriage rate per transfer compared with transferring an untested embryo.
• However, when looking at the chance of a baby per IVF cycle started, PGT-A has not been clearly shown to improve outcomes in younger patients with good ovarian reserve. In older patients and certain other groups, the picture is more nuanced.
• PGT-M is highly effective at avoiding the transfer of embryos affected with the specific condition tested, in appropriately selected couples.
• PGT does not change the chance that a transferred embryo will implant beyond the contribution of selecting a suitable one; uterine factors, hormonal preparation, and overall fertility still matter.

Personalised estimates for your own chance of success are something only your fertility specialist can give, taking into account age, ovarian reserve, sperm quality, prior IVF history, and the specific reason PGT is being considered. Be cautious of any source — clinic or otherwise — that offers a single confident success number for PGT without that context.

Risks and Limitations of the Procedure

PGT is considered safe when performed by experienced embryology and genetics laboratories, but it is not risk-free.

Risks related to the embryo biopsy

• A very small risk of damage to the embryo during biopsy. In experienced laboratories, the risk that a viable embryo will be lost as a direct result of biopsy is low.
• Slightly more embryos may not survive the freezing-thawing process, though vitrification survival rates remain very high overall.

Risks related to the test result

• Misclassification of an embryo as suitable or unsuitable, including the complex situation of mosaic embryos
• An inconclusive result, requiring repeat biopsy or a decision to transfer without a definitive result
• The possibility that no embryo from the cycle is suitable for transfer

Risks related to the IVF cycle itself

The risks of the IVF process — ovarian hyperstimulation, complications from egg retrieval, multiple pregnancy if more than one embryo is transferred — are unchanged by adding PGT. They are discussed in any IVF consultation.

Risks related to interpretation

PGT results are most useful when interpreted by a team that includes a genetic counsellor, embryologist, and fertility specialist. Decisions about borderline or mosaic results should not be made in isolation.

Emotional and Practical Considerations

Common emotional experiences include:

• Relief at having information about embryos before transfer
• Anxiety during the waiting period between biopsy and results
• Grief when fewer embryos than hoped for are suitable, or when none are
• Difficult feelings about decisions on what to do with embryos that are not transferred
• Ethical, religious, or personal questions about embryo selection itself

For couples using PGT-M, the emotional weight can be especially heavy. Many are carriers because of a personal or family history of serious illness. Going through PGT can bring those experiences forward in unexpected ways.

Practical points that often help:

• Plan for the waiting period. Results may take a few weeks. Knowing this in advance can ease the experience.
• Make time for counselling. Both genetic counselling and emotional or psychological support can be valuable. Many fertility centres include access to a counsellor as part of the care pathway.
• Discuss in advance what to do with unsuitable embryos. Options usually include discarding, donating to research where permitted, or continued storage. These decisions are easier to make when not under emotional pressure after results.
• Talk about how much you want to know. Some couples wish to know the sex of suitable embryos for medical reasons related to sex-linked conditions; many other times, sex is not disclosed. Discuss this clearly with your team in line with what is medically appropriate and legally permitted in the country of treatment.

Special Situations

HLA matching

In rare situations, PGT may be combined with HLA tissue matching to identify an embryo that is both unaffected by a serious inherited condition and a tissue match for an existing sibling who needs a stem cell transplant. This is a specialised use of PGT, decided in close consultation with paediatric haematologists, geneticists, and an ethics framework.

Inherited cancer predisposition

Couples in which one parent carries a known cancer-predisposition gene, such as BRCA1, BRCA2, or one of the Lynch syndrome genes, may be offered PGT-M. The decision is personal and often involves discussion of the lifetime risks, available cancer screening, and family values, in addition to standard genetic counselling.

Donor eggs or sperm

When donor eggs or sperm are used, PGT may still be considered — for example, if the donor and the intended parent are both carriers of the same recessive condition identified on carrier screening. Decisions in donor cycles are made together with the fertility centre and, where required, the donor programme.

Recurrent pregnancy loss

Couples with recurrent miscarriage are often investigated for chromosomal causes (including karyotype testing of both partners). When a translocation is found in one partner, PGT-SR may be considered. When no specific cause is found, the role of PGT-A is debated, and decisions are individualised.

Frequently Asked Questions

Is PGT the same as prenatal testing?

No. PGT is done on embryos in the laboratory, before pregnancy. Prenatal testing — such as NIPT (non-invasive prenatal testing), chorionic villus sampling, or amniocentesis — is done during pregnancy. The two are not interchangeable. Even after PGT, prenatal screening and, in some cases, diagnostic testing are usually still recommended.

Does PGT guarantee a healthy baby?

No. PGT reduces the risk of the specific conditions it tests for, but it cannot detect every genetic or developmental condition, and it cannot prevent problems that arise during pregnancy or birth. It is one tool, not a guarantee.

Does the embryo biopsy harm the embryo?

When performed at the blastocyst stage by an experienced embryologist, biopsy has a low risk of harming the embryo. Cells are taken from the trophectoderm, which forms the placenta, not from the inner cell mass that becomes the baby. Survival rates after biopsy and freezing are very high in experienced laboratories.

Why are embryos frozen instead of transferred fresh?

Because genetic analysis takes time, embryos are vitrified after biopsy and transferred in a later, prepared cycle. Frozen embryo transfer is now a standard part of IVF practice and is associated with very good outcomes when handled by an experienced laboratory.

How long does it take to get PGT results?

Most laboratories report results within one to a few weeks of biopsy. For PGT-M with a custom-designed test, the timeline before the cycle can be longer because the test must be developed first.

What happens if no embryo is suitable?

This is one of the harder outcomes of PGT. Depending on the situation, options may include further IVF cycles, the use of donor eggs or sperm, or considering other paths such as adoption. Counselling at this stage is important. Where appropriate, your fertility specialist may also revisit whether PGT-A added value in your specific case.

Is PGT mandatory in IVF?

No. PGT is used in specific clinical situations rather than routinely. Most IVF cycles worldwide do not include PGT. Whether it is right for you is a clinical decision made with your specialist.

Can PGT test for traits like intelligence, appearance, or personality?

No. PGT is used for medical purposes — chromosomal conditions, defined inherited diseases, and structural chromosome rearrangements. It cannot predict complex traits, and using it to attempt to do so is not part of legitimate medical practice.

Will my doctor recommend transferring a mosaic embryo?

Decisions about mosaic embryos are individual and depend on the specific result, the available alternatives, and your preferences after counselling. Practices have evolved as evidence has grown. Some mosaic embryos have led to healthy pregnancies, but the decision involves careful discussion with the fertility specialist and a genetic counsellor.

Conclusion

Preimplantation Genetic Testing is one of the more powerful, and one of the more emotionally weighty, tools in modern fertility care. Used in the right clinical context — a known inherited condition, a chromosomal rearrangement, recurrent miscarriage with a likely chromosomal cause, or specific situations such as advanced maternal age — PGT can help fertility teams and families make more informed decisions about which embryo to transfer.

What PGT cannot do is take away every uncertainty of IVF or pregnancy. It works with the embryos that exist; it cannot create more or change their underlying quality. It reduces specific risks; it does not guarantee a healthy baby. It answers a defined question; it does not screen for everything.

Used thoughtfully, alongside good genetic counselling, careful embryology, and continued prenatal care during pregnancy, PGT can be a meaningful part of the fertility journey for many families. Whether and how it fits your situation is a conversation to have with your fertility specialist and a genetic counsellor, with time to consider the answers before the cycle begins.