PGD or PGS? Understanding the Difference Between These Two Genetic Tests

Oct 25, 2026

IVF

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If your fertility doctor has mentioned PGD or PGS, chances are you have already searched for the difference and come away more confused than before. The two names sound almost identical, the abbreviations keep changing, and some clinics now use entirely different letters, such as PGT-A and PGT-M.

Underneath the alphabet soup, though, the idea is simple: both tests examine the genetic characteristics of an embryo before it is transferred to the uterus. What differs is the question each test is designed to answer, and that matters when deciding whether testing is appropriate for your IVF journey.

PGD and PGS: What the Terms Actually Mean

Preimplantation genetic diagnosis, or PGD, is the older term associated with testing embryos for a specific inherited genetic condition or a known structural chromosome rearrangement. These tests are now generally called PGT-M and PGT-SR, respectively.

Preimplantation genetic screening, or PGS, is the older term for PGT-A. This test assesses whether an embryo has the expected number of chromosomes. Chromosome-number differences can affect implantation and increase the risk of miscarriage, although PGT-A does not guarantee pregnancy or a healthy baby.

PGD vs PGS at a Glance

Feature	PGD (PGT-M / PGT-SR)	PGS (PGT-A)
What it checks	A specific inherited gene condition or structural chromosome rearrangement	Whether an embryo has the expected number of chromosomes
The question it answers	Does this embryo have the specific genetic condition or chromosome risk being tested?	Does this embryo show the expected chromosome number?
Who it may be for	Couples with a known inherited condition or a parent who carries a chromosome rearrangement	Patients considering chromosome screening as part of an individualised IVF plan
What it does not tell you	Other genetic conditions or chromosome-number differences unless these are tested separately	Whether an embryo is free of all genetic conditions or will result in a healthy baby
Key consideration	Can help reduce the risk of passing on a specific known genetic condition	Benefits vary by patient; routine use for every IVF patient has not been established

The Difference Between PGT-A and PGT-M, Explained Simply

Imagine each embryo as a book. PGT-A checks whether the book has the expected number of chapters, not too many and not too few. PGT-M looks for one specific misprint, a known genetic change that runs in the family. PGT-SR checks for structural chromosome rearrangements that may result in missing or duplicated chromosome material.

This is why the tests are not interchangeable. PGT-A does not test for a specific single-gene condition, while PGT-M does not automatically assess the overall number of chromosomes. If more than one genetic concern is relevant, your fertility specialist and genetic laboratory can explain which tests may be appropriate together.

Genetic Testing Before Embryo Transfer: How the Process Works

Embryo creation: Eggs are collected and fertilised in the laboratory. The embryos are cultured until they reach the blastocyst stage, if development allows.
Biopsy: A trained embryologist takes a small sample of cells from the outer layer of the blastocyst. This layer primarily contributes to the placenta.
Freezing: Embryos are commonly frozen while the sampled cells are sent for genetic analysis.
Results: The laboratory reports its findings, and your specialist explains what the results mean, including any limitations or uncertain findings.
Transfer: If appropriate, an embryo is selected for a later frozen embryo transfer based on the test results and your overall treatment plan.

Who May Benefit From Each Test?

PGD: PGT-M or PGT-SR

These tests may be considered when:

Both parents carry a recessive genetic condition, or one parent carries a dominant or X-linked condition.
A previous child or pregnancy was affected by a known genetic condition.
One parent carries a balanced chromosome translocation or inversion that may affect reproductive outcomes.
A serious inherited condition runs in the family and the relevant genetic change has been identified.

PGS: PGT-A

PGT-A may be discussed as part of an individualised IVF plan, including for patients who:

Are of advanced maternal age, when the likelihood of embryo chromosome-number differences increases.
Have experienced repeated pregnancy loss.
Have had previous unsuccessful IVF cycles.
Have a history of a pregnancy affected by a chromosome condition.

These circumstances do not automatically mean that testing is necessary or will improve the chance of a live birth. Your specialist can explain the potential benefits, limitations, costs and alternatives for your specific situation.

Questions Worth Asking Before You Decide

Before agreeing to embryo testing, consider asking your fertility specialist a few direct questions:

Which test are you recommending, and why is it appropriate for my situation?
How could the results change my treatment decisions or embryo selection?
How does the laboratory report mosaic or inconclusive results?
What are the test's limitations, and what can it not detect?
How will testing affect the overall cost and treatment timeline?

Clear answers can help you understand whether testing fits your treatment plan. If the decision feels overwhelming, fertility counselling may also help you explore your concerns and options.

Ask What the Test Will Change Before You Say Yes

PGD and PGS are older terms for different types of embryo testing. PGT-M looks for a specific inherited genetic condition, PGT-SR examines structural chromosome rearrangements, and PGT-A assesses chromosome number. Understanding these distinctions can help you have a more informed discussion with your fertility specialist about whether testing belongs in your treatment plan.

Goral Gandhi, a clinical embryologist with over 25 years of experience in IVF, offers guidance on genetic testing options in Mumbai. Understanding the purpose and limitations of each test can help you make a decision suited to your circumstances.

Have questions about genetic testing before embryo transfer? Book a consultation to discuss your options with the fertility team.

Frequently Asked Questions

Is PGS the same thing as PGT-A?

Yes. PGS is the older name for PGT-A, which assesses the number of chromosomes in an embryo. PGD is an older umbrella term associated with tests now generally called PGT-M and PGT-SR, which examine specific genetic conditions and structural chromosome rearrangements.

Does a normal PGT-A result guarantee a healthy baby?

No. PGT-A assesses chromosome number in the sampled embryo cells. It cannot rule out every genetic condition, birth defect or future health problem, and it does not guarantee implantation, pregnancy or a live birth. Your specialist can explain the test's limitations and whether other screening or diagnostic tests may be appropriate.

What does a mosaic embryo result mean?

A mosaic embryo result means that testing has detected a mixture of cells with different chromosome findings in the sample. The result may not represent every cell in the embryo. The implications for pregnancy vary, and the evidence has limitations. A fertility specialist and genetic counsellor can help explain the findings and discuss possible options before a transfer decision is made.

Goral Gandhi

Goral Gandhi is the Founder and Scientific Director at Global Fertility Solutions, Mumbai, with more than 25 years of experience in IVF, embryology and assisted reproductive technology (ART). She is a consultant clinical embryologist who has worked across a wide range of infertility cases. Her writing aims to help couples understand each stage of fertility treatment, including the emotional side of the process.