What does PGT-A test, and what does it not test?
PGT-A, preimplantation genetic testing for aneuploidy, counts an embryo's chromosomes. A typical human embryo has 46 chromosomes in 23 pairs. An embryo with an extra or missing chromosome is called aneuploid. Most aneuploid embryos fail to implant or end in miscarriage, and a few result in a pregnancy with a chromosome condition such as Down syndrome.
PGT-A was formerly called PGS, preimplantation genetic screening. The two names describe the same test.
What PGT-A does:
- Reports whether each tested embryo has the expected number of chromosomes.
- Reports the sex chromosomes, as a byproduct of the count.
- Flags some large missing or extra pieces of chromosomes, depending on the lab.
What PGT-A does not do:
- It does not test for single-gene conditions, such as cystic fibrosis. That is PGT-M.
- It does not detect structural birth defects, such as a heart defect.
- It does not guarantee a healthy baby. It is a screening test with a small error rate, and it looks at a few cells, not the whole embryo.
Think of PGT-A as one more piece of information about an embryo, alongside its grade and the day it reached blastocyst. Prenatal screening during the pregnancy is still recommended.
How is the biopsy done, and is it safe for the embryo?
The biopsy is done on a blastocyst, an embryo at day 5 to 7. By then it has two parts: the inner cell mass, which becomes the baby, and the trophectoderm, the outer layer that becomes the placenta.
An embryologist uses a fine glass needle and a laser to remove 5 to 10 trophectoderm cells. The inner cell mass is not touched. The embryo is then vitrified, a fast-freezing method, while the cells go to a genetics lab. Results take 1 to 2 weeks.
A trophectoderm biopsy at the blastocyst stage is the method most clinics use today. It is considered low risk for the embryo when performed by an experienced lab, though no procedure on an embryo is entirely without risk. Ask your clinic about its biopsy experience and how its biopsied embryos survive thawing.
In surrogacy, the freezing step costs nothing extra in time. Embryos for a gestational surrogate are almost always frozen anyway, because the legal agreement and escrow must be in place before the surrogate starts medication. See IVF for surrogacy.
What do euploid, aneuploid and mosaic mean?
Each tested embryo comes back in one of three categories, plus a fourth for when the test does not work.
| Result | What it means | Typical clinic transfer policy | Counseling required | What the surrogacy agreement says |
|---|---|---|---|---|
| Euploid | Expected chromosome count in the cells tested | Transfer first, one at a time | Not usually | Your attorneys write which embryos the surrogate agrees to receive |
| Aneuploid | Extra or missing chromosome in the cells tested | Not transferred at most clinics | Available if you want to understand the result | Not usually transferred; your attorneys write the terms |
| Mosaic | A mix of cells with normal and abnormal counts | Case by case, after counseling, at clinics that allow it | Genetic counseling before any transfer | Must be stated if the surrogate agrees to a mosaic transfer |
| No result | The sample could not be read | Rebiopsy, or transfer untested, by clinic policy | Discuss with the clinic | Your attorneys write the terms |
A PGT-A result for each embryo considered for transfer is listed in the embryo disclosure in the PLS surrogacy agreement. Whether the PLS agreement template includes an embryo disclosure clause is pending confirmation with Nancy.
Why do agencies and surrogates prefer euploid transfers?
Most surrogacy clinics and agencies recommend transferring euploid embryos. PLS recommends euploid transfers. There are three reasons.
- Lower miscarriage risk. Chromosome problems are the most common cause of early miscarriage. Transferring a euploid embryo lowers that risk.
- Fewer transfers that are unlikely to succeed. Every transfer asks the surrogate to complete weeks of estrogen and progesterone, travel to your clinic and wait for a pregnancy test. A surrogate is a partner in your family's journey, and it is fair to her to transfer the embryo most likely to work.
- A clearer plan. When you know how many euploid embryos you have, you know how many transfers you can plan for, and whether you may need another retrieval.
There is also a cost argument. Each transfer adds surrogate compensation, travel, monitoring and clinic fees. Avoiding a transfer that will not succeed saves money and months.
Does PGT-A make sense with young donor eggs?
It depends, and ASRM is clear that PGT-A is not required for every patient.
The rate of aneuploidy rises with the age of the egg provider. That is why the benefit of PGT-A is strongest for older egg providers. With young donor eggs, the aneuploidy rate is lower, but it is not zero. Some embryos from a young donor will still be aneuploid.
The case for testing with young donor eggs in surrogacy usually rests on the surrogate. A failed transfer or early loss costs her weeks of medication and an emotional toll, and it costs you months. Many families decide that is worth avoiding. Others, with many good-quality embryos from a young donor, choose not to test. Your reproductive endocrinologist can help you weigh it, and the donor screening page covers what is known about a donor before her eggs are used.
If two intended fathers are each creating embryos, the same reasoning applies to each group of embryos. See whose sperm to use.
Can mosaic embryos be transferred to a surrogate?
Sometimes. A mosaic result means the lab found a mix of cells with normal and abnormal chromosome counts in the biopsy. Some mosaic embryos implant and lead to healthy births. They are, on average, less likely to implant and more likely to miscarry than euploid embryos.
Three things must be true before a mosaic transfer to a surrogate:
- Genetic counseling. You meet with a genetic counselor to understand the specific result, and the surrogate needs a clear explanation of it too.
- Clinic approval. Not every clinic transfers mosaic embryos, and those that do have their own rules about which ones.
- The surrogate's agreement, in writing. Some surrogates will agree to a mosaic transfer after counseling. Others will not. The agreement must say which embryos she has agreed to receive.
PLS discusses mosaic embryos at matching, so you and the surrogate know where each of you stands before the agreement is drafted. Prenatal diagnostic testing during the pregnancy is often recommended after a mosaic transfer. The clinic decides.
What does PGT-A cost and add to the timeline?
PGT-A is paid to your fertility clinic and its genetics lab, not through PLS. Clinics and labs typically charge a biopsy fee for each embryo and a lab fee for each batch. Prices vary widely. Ask your clinic for its current schedule in writing.
On timeline, PGT-A adds the 1 to 2 weeks for results after the last embryo is frozen. In surrogacy, that usually runs in parallel with matching, legal work and escrow, which take longer. Legal and medical foundations take 1 to 2 months on their own. In most journeys, PGT-A does not delay the transfer.
If a test returns no result and an embryo needs a second biopsy, that adds another thaw, biopsy and refreeze, and more time.
How is PGT-A different from PGT-M and PGT-SR?
All three use the same kind of embryo biopsy. They look for different things.
- PGT-A counts chromosomes. It is a general screen.
- PGT-M, for monogenic conditions, tests for a specific single-gene condition that runs in the family, such as cystic fibrosis or sickle cell disease. It requires work in advance to build a test for that family's specific mutation, which can take weeks or months.
- PGT-SR, for structural rearrangements, is used when a parent carries a chromosome rearrangement, such as a translocation, that can lead to unbalanced embryos.
PGT-M and PGT-SR are often done together with PGT-A on the same biopsy. If you or your donor carries a known genetic condition, tell your clinic early, so the test is ready when embryos are created.
This page gives general information. It is not medical advice. Whether to test, and which embryos to transfer, is a decision you make with your clinic.
Sources
- ASRM and SART Practice Committees, committee opinion on the use of preimplantation genetic testing for aneuploidy. Title and year of the current ASRM committee opinion on PGT-A to be confirmed before publishing. https://www.asrm.org/
- ASRM, guidance on the clinical management of mosaic PGT-A results.
- SART (Society for Assisted Reproductive Technology), clinic outcome reports. https://www.sart.org/
- CDC (Centers for Disease Control and Prevention), national assisted reproductive technology data. https://www.cdc.gov/art/