PGT-A vs PGT-SR: What’s the Difference for Donor Egg Embryos?

If you are using donor eggs and considering genetic testing of embryos, you may come across two common terms: PGT-A and PGT-SR. Both are types of preimplantation genetic testing (PGT), but they look for different things. PGT-A screens embryos for an abnormal number of chromosomes (aneuploidy), while PGT-SR checks for structural rearrangements of chromosomes, such as inversions or translocations. This article explains the difference, how each applies to donor egg cycles, and what to discuss with your clinic.

At a Glance: PGT-A vs. PGT-SR

• PGT-A (Preimplantation Genetic Testing for Aneuploidy) counts chromosomes to detect extra or missing chromosomes.
• PGT-SR (Preimplantation Genetic Testing for Structural Rearrangements) looks for changes in chromosome structure, like deletions, duplications, inversions, or translocations.
• PGT-A is often considered for embryos from donor eggs, especially if the donor is older or if you want to reduce the chance of transferring an embryo with an abnormal chromosome number.
• PGT-SR is usually recommended when one or both genetic parents (including the egg donor) are known carriers of a balanced structural rearrangement.
• Neither test guarantees a pregnancy or a healthy baby; they are screening tools that provide information to help guide embryo selection.

What Is PGT-A?

PGT-A, also called preimplantation genetic screening (PGS), is a test that checks embryos for the correct number of chromosomes. Humans typically have 46 chromosomes in each cell, arranged in 23 pairs. An embryo with an abnormal number of chromosomes—such as an extra copy of chromosome 21 (which causes Down syndrome) or a missing chromosome—is said to be aneuploid. Most aneuploid embryos fail to implant or result in miscarriage, and some can lead to conditions like trisomy 18 or Turner syndrome.

PGT-A is performed on a small number of cells taken from a blastocyst (a day-5 or day-6 embryo) before transfer. The test can identify embryos that are likely to have a normal chromosome count (euploid) versus those that are aneuploid. By selecting a euploid embryo for transfer, the goal is to improve the chances of implantation and reduce the risk of miscarriage due to chromosomal abnormalities.

Why Consider PGT-A with Donor Eggs?

When using donor eggs, the age of the egg donor is a key factor. Younger donors (typically under 35) have a higher proportion of chromosomally normal eggs, but the risk of aneuploidy increases with maternal age. Even with a young donor, some embryos may still be aneuploid. PGT-A can help identify which embryos are most likely to be viable, potentially reducing the number of transfer cycles needed.

However, PGT-A is not mandatory for all donor egg cycles. Some clinics may recommend it based on the donor’s age, the number of embryos available, or your personal history. It is important to discuss the potential benefits and limitations with your fertility specialist.

What Is PGT-SR?

PGT-SR is a test designed for couples or donors who carry a structural rearrangement of chromosomes. Structural rearrangements include:

• Translocations: A piece of one chromosome breaks off and attaches to another chromosome. This can be balanced (no genetic material is lost or gained) or unbalanced (extra or missing genetic material).
• Inversions: A chromosome segment breaks off, flips, and reattaches in the reverse orientation.
• Deletions and duplications: A part of a chromosome is missing or duplicated.

Carriers of balanced rearrangements often have no health problems themselves, but they have a higher risk of producing embryos with unbalanced chromosomes, which can lead to miscarriage, stillbirth, or a child with disabilities. PGT-SR identifies embryos that have a normal or balanced chromosome structure, so that only those are considered for transfer.

When Is PGT-SR Relevant for Donor Eggs?

PGT-SR is relevant when the egg donor is a known carrier of a structural rearrangement. In such cases, the donor’s eggs may carry the rearrangement, and embryos created with those eggs could be affected. If the donor has not been tested for chromosomal rearrangements, PGT-SR is not typically performed. It is also important to note that PGT-SR is different from PGT-M, which tests for specific single-gene disorders (like cystic fibrosis or sickle cell anemia).

If you are using donor eggs, your clinic will usually have information about the donor’s genetic screening results. If the donor is a carrier of a balanced translocation or inversion, PGT-SR may be recommended. However, this is a specific situation, and most donor egg cycles do not involve PGT-SR unless there is a known indication.

Key Differences Between PGT-A and PGT-SR

Aspect	PGT-A	PGT-SR
What it detects	Abnormal number of chromosomes (aneuploidy)	Structural changes in chromosomes (e.g., translocations, inversions)
Main purpose	Screen for embryos with the correct chromosome count	Identify embryos with normal or balanced chromosome structure
Who might need it	Anyone, but often considered for older egg donors or when many embryos are available	Only when a genetic parent (including donor) is a known carrier of a structural rearrangement
How it works	Counts chromosomes in a biopsy sample	Analyzes chromosome structure, often using similar biopsy techniques
Limitations	Does not detect all genetic abnormalities; may have false positives/negatives; does not guarantee a healthy baby	Only relevant for specific structural issues; does not screen for aneuploidy or single-gene disorders

Is PGT-A Recommended for Donor Egg Cycles?

There is no universal recommendation for PGT-A in donor egg cycles. Some clinics offer PGT-A as an option, while others may not recommend it routinely, especially if the donor is young and the embryo quality is good. The decision depends on several factors:

• Donor age: Older donors have a higher risk of aneuploidy, so PGT-A may be more beneficial.
• Number of embryos: If you have many embryos, PGT-A can help prioritize which to transfer first.
• Previous pregnancy losses: If you have experienced recurrent miscarriages, PGT-A might provide additional information.
• Personal preference: Some patients want as much information as possible before transfer.

It is essential to have a detailed discussion with your fertility team about the potential benefits, risks, and costs of PGT-A in your specific situation. They can help you weigh the evidence and make an informed choice.

Limitations of PGT-A and PGT-SR

Both tests have limitations that you should understand:

• Not diagnostic: PGT is a screening test, not a guarantee. A normal result does not ensure a healthy baby, and an abnormal result does not always mean the embryo cannot result in a live birth.
• Mosaicism: Embryos may have a mix of normal and abnormal cells (mosaicism). PGT-A may report a mosaic result, which is challenging to interpret.
• Technical errors: There is a small risk of false results due to the biopsy or laboratory process.
• Embryo damage: The biopsy procedure carries a small risk of damaging the embryo, although this is rare.
• Not for all genetic conditions: PGT-A does not screen for single-gene disorders or structural rearrangements; PGT-SR only looks for structural issues.

It is also important to note that PGT does not eliminate the risk of miscarriage or birth defects entirely. Many other factors contribute to a successful pregnancy.

Questions to Ask Your Clinic

Before deciding on PGT-A or PGT-SR, consider asking your clinic the following questions:

• What is the age of the egg donor, and does that affect the recommendation for PGT-A?
• Has the donor been tested for chromosomal rearrangements? If so, what were the results?
• How many embryos are expected, and how might that influence the decision?
• What is the accuracy of the PGT test you use, and how are mosaic results handled?
• What are the costs of PGT-A and PGT-SR, and are they included in any package?
• How long does testing take, and how does that affect the timing of a frozen embryo transfer?
• What are the risks of the biopsy procedure?

Next Steps

If you are considering PGT with donor eggs, here is a simple checklist to guide you:

1. Review the donor’s genetic screening results with your clinic.
2. Discuss whether PGT-A is recommended based on donor age and embryo count.
3. Ask if PGT-SR is needed if the donor is a known carrier of a structural rearrangement.
4. Understand the limitations and what a normal result does and does not mean.
5. Make an informed decision that aligns with your values and medical history.

For more information about PGT in Thailand, visit our PGT in Thailand page. You can also explore our guides and FAQ sections for additional insights.

Frequently asked questions

Is PGT-A necessary when using donor eggs?

PGT-A is not always necessary. It may be recommended based on the donor's age, the number of embryos available, or your medical history. Discuss with your clinic to determine if it is beneficial for your situation.

Can PGT-SR be done on embryos from donor eggs?

Yes, PGT-SR can be performed if the egg donor is a known carrier of a structural chromosomal rearrangement. It is not typically done unless there is a specific indication.

What is the difference between PGT-A and PGT-M?

PGT-A screens for abnormal chromosome numbers, while PGT-M tests for specific single-gene disorders. They are different tests and are used for different purposes.

Does PGT-A guarantee a healthy baby?

No. PGT-A is a screening test that reduces the risk of transferring an embryo with an abnormal chromosome count, but it does not guarantee a pregnancy or a healthy baby. Other genetic and non-genetic factors also play a role.

What are the risks of embryo biopsy for PGT?

Embryo biopsy is generally safe, but there is a small risk of damaging the embryo. The procedure is performed by experienced embryologists to minimize this risk.

Continue your research

• PGT in Thailand overview
• PGT and IVF guides
• Frequently asked questions

Medical information notice: This article is educational and does not replace individual assessment, diagnosis, genetic counselling or treatment advice from a licensed clinician. Provider services, availability, fees and policies should be verified directly before booking.