What is PGT-A, and what does it examine and not examine? The evidence per ESHRE, ASRM and HFEA assessments, its limits and questions for your doctor.
PGT-A is a genetic screening method in which the number of chromosomes is examined in a few cells taken from an embryo obtained through IVF. According to a current assessment prepared for Europe, it is not recommended for routine clinical use; the data available show only a limited improvement in live birth. The test gives no assurance about pregnancy or birth. For some people it is considered as an option that can be discussed with the doctor; your doctor and genetic counsellor assess whether it is suitable for you.
What does PGT-A look for in chromosome number, and what does PGS mean?
An embryo whose chromosome number differs from what is expected is called aneuploid. PGT-A looks for this difference. PGS (preimplantation genetic screening), its old name, is another name used today for the same test. Individual genes are not the subject of this test.
What does PGT-A not examine?
The PGT-M and PGT-SR tests, done for single-gene disease and for a structural chromosome change known in the family, serve different purposes. Even if the result of PGT-A is "normal", it gives no information on these headings.
How does the process work?
PGT-A is an extension of an IVF or ICSI cycle. Your embryos are grown in the laboratory up to the blastocyst stage, a small number of cells are taken from the outer layer, and this sample is examined in the genetic laboratory. The technique of taking the cells is a separate subject. According to the current assessment on this subject, if the result will take days after blastocyst biopsy, freezing the embryos is mostly necessary. When the result arrives, which embryo will be transferred and when is decided with your doctor.
For which reasons can PGT-A be considered for you?
In the current expert recommendations on this subject, the headings listed for PGT-A are advanced maternal age, recurrent implantation failure, recurrent miscarriage and severe male factor. The same recommendations emphasise that each centre determines its own criteria and that the test is not standard care. This list does not mean that a benefit has been shown in the groups mentioned. If the test has been recommended to you because of age or a history of miscarriage, it is sensible to talk with your doctor about the reasoning.
What does the European assessment say about routine use?
The assessment of add-on practices prepared for Europe summarises the controlled studies on this subject. The live birth results are mixed: in some studies in favour of PGT-A, in one in favour of the control, and in others no difference was found. It is stated that in some studies a lower frequency of miscarriage was reported, but how this is reflected in live birth is not clear. The idea that it shortens the risk of miscarriage or the time to pregnancy in older women rests on later analyses and requires further research.
Does the American assessment support universal screening?
According to another current view on this subject, the value of PGT-A as universal screening for all IVF patients has not been shown. The same view writes that it may have a useful role in advanced maternal age, particularly in women with good ovarian reserve, and that there is a biologically reasonable justification in recurrent pregnancy loss, but that there is no proof of definite benefit.
What does the assessment in the United Kingdom say about effectiveness?
The regulator in the United Kingdom states that at this stage there is no controlled-study evidence showing that PGT-A is effective in increasing the chance of a baby for most patients. It adds that there is some evidence that it reduces miscarriage, but that this remains in the nature of secondary findings. According to this body, practices without strong evidence should be offered only in a research setting.
These three assessments have their own classification language, and these statements cannot be translated into one another. The European assessment does not recommend routine use, the American assessment says its value as universal screening has not been shown; for advanced age and recurrent pregnancy loss it is stated that there is no definite evidence. The assessment in the United Kingdom reports that there is no controlled-study evidence of effectiveness for most patients.
Can the test give a wrong result?
The current assessment on this subject states that different results have been reported between laboratories, particularly in the diagnosis of mosaicism, and that there is a lack of standardisation. A warning is also given that viable embryos may have been discarded because of analytical errors. The regulator in the United Kingdom also writes that the test can give an erroneous result and that healthy embryos may be discarded.
Does the test affect the number of embryos that can be transferred and the time taken?
According to the current assessment on this subject, PGT-A often reduces the number of embryos that can be transferred. The additional testing stage can also lengthen the time to pregnancy.
Can taking cells affect the embryo?
Assessments on this subject report that taking the cells may harm the embryo and hinder its development. Another assessment also says that the effect of trophectoderm biopsy is not well understood. Follow-up studies conducted after blastocyst biopsy have not shown an adverse pregnancy outcome; a small increase in intrauterine growth restriction may, however, require separate examination.
How is a mosaic result assessed?
In some embryos some of the cells may carry the expected chromosome number and some a different one; this is called a mosaic result. According to the current recommendations on this subject, mosaicism detected in the outer layer may not reflect the structure or viability of the whole embryo, and it is recommended that low-level mosaic embryos not be discarded. The classification can vary from laboratory to laboratory. You should talk separately in genetic counselling about what the result means.
What does an embryo whose chromosome number comes out as expected mean?
This result relates only to chromosome number. It does not exclude single-gene diseases, structural problems in the chromosome or complications during pregnancy and birth. Standard methods may be unable to detect abnormalities below the resolution of the platform and low-level mosaicism; the current assessment on this subject clearly records these technical limits.
What questions can you ask your doctor before deciding?
- Why is this test being recommended for me — is the reason my age or my history?
- How does my plan change if I do not have the test?
- What is in the result report, and how are mosaic results reported?
- What is envisaged if there is no embryo that can be transferred?
- Is the test subject to an additional fee?
Frequently Asked Questions
If a pregnancy occurs after PGT-A, is prenatal testing still needed?
Yes, the current recommendations on this subject call for counselling and options on prenatal diagnosis to be offered to every woman who conceives with PGT. The test gives preliminary information at the embryo stage for chromosome number; it does not take the place of diagnostic methods in pregnancy. The doctor who follows your pregnancy determines the suitable method and timing.
What happens if all the embryos come out "different from expected"?
The recommendations on this subject ask that the possibility of having no embryo that can be transferred be discussed during counselling. Another recommendation asks that clinics have a written policy on the fate of embryos whose chromosome number comes out different from expected. For this reason it is reasonable to ask, before starting the test, what such a result would mean and what would happen to the embryos.
Is not having PGT-A an option?
Yes. According to the current view on this subject, the consent process should include discussion of the risks, benefits and limitations and the option of not having PGT-A.
Can I ask the centre for information on the likelihood of the test giving a wrong result?
According to the recommendations on this subject, estimates of misdiagnosis should be given to you when you ask. You can ask the centre or laboratory where the test will be done for this information.
Does PGT-A look for all genetic diseases?
No. PGT-A examines the chromosome number. PGT-M, aimed at single-gene diseases, and PGT-SR, aimed at a structural change known in the family, are different tests.
If the PGT-A result is normal, is pregnancy certain?
No. The result gives information only about the chromosome number examined; it gives no assurance about pregnancy or birth.
Does its being recommended because of advanced age mean the benefit is certain?
No. That this group is mentioned in the recommendations does not mean that a benefit has been shown. Although the assessments discuss a possible role at an advanced age, they state the limits of definite evidence.
Can PGT-A reduce the number of embryos that can be transferred?
Yes. According to the current assessment, it often reduces the number of embryos that can be transferred. The additional testing stage may also lengthen the time to pregnancy.
Does a mosaic result show the structure of the whole embryo?
Not always. Mosaicism detected in the outer layer may not reflect the structure or viability of the whole embryo; the result is assessed in genetic counselling.
Does a low-level mosaic result mean the embryo will be discarded?
No. The current recommendations advise that low-level mosaic embryos not be discarded. How the result is classified may vary with the laboratory.
May freezing the embryos be necessary after the biopsy?
Yes. If the result will take days after blastocyst biopsy, freezing is mostly necessary. You decide the time of the transfer with your doctor.
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