Why PGT-A and PGT-M Answer Different Genetic Questions

Two embryos can both receive a “normal” result and still be tested for completely different reasons. That is the simplest way to understand why PGT-A and PGT-M are not interchangeable: they are built to answer different questions, using different laboratory designs, and each has its own blind spots.

PGT-A asks whether an embryo’s cells show the expected number of chromosomes. PGT-M asks whether an embryo inherited a specific genetic variant that is already known to run in a family. Neither test is a general certificate of embryo health, and neither guarantees a pregnancy or a healthy child.

What PGT-A is designed to look at

PGT-A stands for pre-implantation genetic testing for aneuploidy. Aneuploidy means an abnormal number of chromosomes — for example, an extra copy or a missing copy of a particular chromosome. According to the HFEA’s patient information on PGT-A, the test assesses chromosome number in cells sampled from an embryo.

That scope matters. PGT-A is a genome-wide count, not a search for a named condition. It does not know that a particular family carries a variant in a particular gene, and it is not designed to detect one. A PGT-A report may flag an abnormal chromosome count, or it may describe a mosaic finding — an intermediate result inferred from the biopsy sample. As the ASRM committee opinion on mosaic PGT-A results explains, technical factors can contribute to such findings, and a sampled result does not establish the cell composition of the whole embryo.

So a reassuring PGT-A result means the biopsy did not show an abnormal chromosome count. It says nothing about a single-gene condition your family may carry.

What PGT-M is designed to look at

PGT-M stands for pre-implantation genetic testing for monogenic disorders — conditions caused by a variant in a single gene. The HFEA’s information on PGT-M and PGT-SR describes PGT-M as testing for a known monogenic condition, and notes that a treating team and genetic counsellor should discuss the relevant history, test design and suitability.

That word “known” carries a lot of weight. PGT-M is not a scan for unknown problems. It is a targeted test built around a specific variant — or, in some designs, around linked markers near that variant — that has already been identified through prior testing of the family or the parents. The laboratory needs that information to build a reliable assay before it can report whether an embryo inherited the variant.

Because the test is targeted, a PGT-M result tells you about that one condition. It does not confirm that the embryo’s chromosome number is normal across the genome. A separate test would be needed for that question.

A side-by-side view

Question	PGT-A	PGT-M
What is being examined?	Chromosome number across the genome	Inheritance of a known familial variant
Does it need prior family testing?	No — it is not built around a family variant	Yes — the assay depends on known variant information
Can it rule out a known single-gene condition?	No	It is designed to address that condition, within the limits of the assay
Can it confirm normal chromosome number?	It assesses this, with limitations	No
Typical report language	Aneuploid, euploid, mosaic, no result	Variant present, variant absent, inconclusive, no result

PGT-SR is a third category, used when a parent carries a chromosomal structural rearrangement rather than a single-gene variant. It is a distinct question again, and the HFEA source above groups it with PGT-M only to distinguish their separate purposes.

Why a normal PGT-A result does not cover a family condition

This is the point that most often causes confusion. A couple who knows they carry a variant for a single-gene condition may assume that a “normal” PGT-A result means the embryo is unaffected. It does not. PGT-A was never looking at that gene. The variant could be present in an embryo whose chromosome count looks normal on biopsy.

The reverse is also true. An embryo reported as unaffected on PGT-M may still have an abnormal chromosome count, because PGT-M does not assess that. Each result answers its own question and leaves the other open.

When the two tests are combined

Some families are advised to consider both tests together: PGT-M for the known condition and PGT-A for chromosome number. Combining them can address two separate questions in one biopsy, but it is not automatically the right choice for everyone.

It is worth asking your clinician what the combination adds in your situation, and what it costs — in biopsy material, laboratory analysis, time and counselling. A combined approach also means two sets of results to interpret, each with its own uncertainty. The decision belongs with your treating team and genetic counsellor, who can weigh your history and the test design.

Limits that apply to both tests

• Sampling, not the whole embryo. Both tests work from a small biopsy. A result describes the sample, and the ASRM opinion stresses that a sampled result does not establish the whole embryo’s cell composition.
• Mosaicism. Intermediate PGT-A findings can be technically influenced and need careful discussion rather than a simple label.
• No-result or inconclusive outcomes. Some biopsies do not yield a clear answer, and a repeat may or may not be possible.
• Variants of uncertain significance. A variant’s link to disease is not always clear, which can complicate PGT-M design.
• Residual risk. Neither test removes all uncertainty about an embryo or a pregnancy.
• Biopsy itself carries a risk. The HFEA’s PGT-A questions and answers note that embryo biopsy can cause damage and prevent further development.

Neither test guarantees a successful pregnancy or a healthy child, and neither is a required step for every patient. Whether testing is appropriate is an individual clinical decision.

Preparing for a useful conversation about test design

You can ask your clinician or genetic counsellor directly about diagnosis, options, risks, reports and your individual chances. A few preparation points help that conversation stay concrete:

• Bring any prior genetic reports — the family variant, who was tested, and what the laboratory found.
• Ask which test is being proposed and exactly what question it answers.
• Ask what the report will and will not tell you, including how no-result and mosaic findings are handled.
• Ask whether a combined approach is being considered and why.
• Keep a dated record of what was discussed and what remains open.

If you are comparing clinics or gathering information before a consultation, our guide to PGT-A, PGT-M and PGT-SR differences for international patients sets out the categories in more detail, and how to read PGT-A results covers report language. For families weighing a known condition, PGT-M versus PGT-A for single gene disorders goes further into that comparison.

Coordination support can help with the practical side: organising records and prior genetic reports, requesting a remote consultation, planning travel around confirmed appointments, and arranging a records handover before you return home. Clinicians interpret results and make treatment decisions; the coordinating service does not diagnose or prescribe.

Frequently asked questions

If I have PGT-A done, do I still need PGT-M for a known family condition?

They answer different questions. PGT-A assesses chromosome number and is not designed to detect a specific single-gene variant, so a normal PGT-A result does not rule out a known familial condition. Whether PGT-M is appropriate for you is a clinical decision for your treating team and genetic counsellor.

Can PGT-M tell me whether an embryo's chromosomes are normal?

No. PGT-M is targeted at a known variant and does not confirm chromosome number across the genome. If both questions matter in your situation, ask your clinician whether a combined approach is being considered and what it involves.