What is a mosaic embryo, and what does mosaic on a PGT report mean? The decision process per ESHRE, ASRM and HFEA approaches, and questions to ask.
"Mosaic embryo" written in a genetic screening (PGT) report means that, in the sample taken from the embryo, cells with the expected chromosome number and cells with an unexpected one are present together. That is, the embryo does not clearly fall into one of the groups "chromosomes as expected" or "chromosomes different from what is expected". The sample taken may not reflect the whole embryo. There is also not yet a method that can reliably predict what a mosaic result means for pregnancy and birth. Because the data on this are limited, the decision is made according to the centre's own policy and genetic counselling specific to you. Seeing this word in the report may make you anxious. You can talk in genetic counselling about what the result means and which options exist before the decision.
What do mosaicism and different groups of cells mean?
Mosaicism means that more than one group of cells with different chromosome structure is present in a single embryo. You are expected to be told in counselling that this is a natural feature that can be seen in embryos before they implant in the uterus. That is, the word "mosaic" is not in itself the name of a disease; it is only a description showing that the cells in the embryo have come out different from one another.
How does a mosaic result differ from other chromosome results?
In PGT-A and PGT-SR reports that examine the chromosomes, results fall into three groups: those in which the chromosome number is as expected in all the cells of the sample, those in which it is different from what is expected in all of them, and mixed ones. A mosaic result falls into this third group. It is not found right to count a mosaic embryo as "having a disordered chromosome number" only for this reason. But it cannot be said to be free of problems either; the uncertainty remains. A mosaic result is not the name of a disease.
Does the sample taken represent your whole embryo?
For the test, a few cells are taken from the outer layer of the blastocyst (the trophectoderm). The mosaicism detected in this small sample may not always reflect the chromosome structure of the whole embryo. Centres are expected to state this limit in their reports.
Does the classification of low and high level vary between laboratories?
Expressions such as "low level" and "high level" are used in reports; these are a distinction made according to the proportion of different cells in the sample. It is recommended to avoid giving exact percentage values, because centres may use different thresholds for this distinction. That is, a result called "low level" in one laboratory may be assessed in a different group in another laboratory.
Technical interpretation also affects the result. For example, an inconsistent result in more than one cell may arise from a technical error or from true mosaicism. Studies in this area draw attention to the fact that there may be inconsistency between laboratories in the diagnosis of mosaicism and that healthy embryos may have been wrongly discarded because of analytical errors.
What options can be discussed for your mosaic embryo?
Routes such as storing by freezing, considering transfer or not using the embryo are discussed for an embryo reported as mosaic. Each IVF centre is expected to have its own policy on the transfer, storage and fate of mosaic embryos. For this reason which option is possible may vary from centre to centre and according to the rules in force; it is good to learn your centre's approach before deciding.
What happens if you have other embryos with the chromosome number as expected?
If you also have embryos from the same cycle with the chromosome number as expected, using them first is regarded as a reasonable option left to your preference. This is not a rule, but an assessment that varies from person to person.
What do the European recommendations say about low-level mosaic embryos?
According to current guidelines in Europe, discarding an embryo reported as low-level mosaic only for this reason is not found acceptable. The same guidelines also oppose automatically counting a mosaic embryo as "having a disordered chromosome number", and ask for counselling specific to the person for the decision.
According to the American view, is it possible to predict risk reliably?
According to the current view in the United States, there is at present no proven system that ranks mosaic embryos or reliably predicts the risk before and after birth. Those considering the transfer of a mosaic embryo are advised to see a genetic counsellor; it is also stated that risk prediction is limited.
How does the assessment in the United Kingdom handle reports of healthy births?
The regulator in the United Kingdom states that the chance of pregnancy may be lower with mosaic embryos, but that healthy births have also been reported after transfer. The testing, reporting and transfer practices of centres may differ from one another; you need to ask your centre about this.
Although the emphasis differs between regions, nothing contradicts anything else: reliable information is still limited, and the decision is made together with counselling specific to the person and the centre's policy. Reports of healthy births do not mean a guarantee.
Which counselling and testing options are offered after a mosaic embryo is transferred?
After a mosaic embryo has been transferred, it is recommended that genetic counselling and prenatal testing options be offered to you.
What questions can you ask before deciding?
- By which expressions and by which criterion are mosaic results classified in the report?
- Does this centre transfer mosaic embryos, store them by freezing, or follow another route?
- Do I have other embryos with the chromosome number as expected, and how is the ranking done?
- If a mosaic embryo is transferred, what kind of monitoring is planned during the pregnancy?
- What information on mosaic results is in the consent form?
- When can I meet with genetic counselling?
Deciding in uncertainty can be tiring. For this reason it is good to ask for a counselling consultation before deciding and to find answers to your questions.
Frequently Asked Questions
Can a mosaic result be confirmed by re-biopsy?
Re-biopsy is not recommended in order to confirm mosaicism; there is no evidence that such a procedure has additional diagnostic value. That is, a mosaic report is not an intermediate stage to be clarified with a second sample; the result you have is assessed as it stands in counselling.
If a pregnancy occurs from a mosaic embryo, is amniocentesis essential?
It is recommended that prenatal diagnosis counselling be offered to everyone who conceives after PGT. Low-level mosaicism detected in the outer layer is not regarded on its own as a sufficient reason for an invasive prenatal test (such as amniocentesis). It is also stated that there are not enough data on this and that opinions are debated. That is, amniocentesis is not an obligation; the decision is made in counselling, together with the doctor who follows you.
Is the possibility of a mosaic result discussed before the test is done?
Yes, counselling must be done before the PGT test starts. In this consultation you are expected to be told the technical and biological limits of detecting mosaicism and the centre's policy on transferring and freezing mosaic embryos. The laboratory's approach to mosaicism should also be stated in the consent form. You can ask your centre for this information before the test starts.
Is "mosaic" in the report the name of a disease?
No. It is a description of groups of cells with different chromosome structure. On its own it is neither the name of a disease nor a guarantee that there is no problem.
Does a mosaic result show the whole embryo for certain?
No. The small sample taken from the outer layer may not always reflect the chromosome structure of the whole embryo.
Does low-level mosaic mean the same thing in every laboratory?
No. Centres may use different classification thresholds. A low-level result in one laboratory may be assessed differently in another laboratory.
Can an inconsistent result arise from a technical error?
Yes. An inconsistent result from different cells can be due to a technical error or to true mosaicism. There may be differences of interpretation between laboratories.
Should a low-level mosaic embryo be discarded only for this reason?
The European recommendations state that this is not acceptable. The same recommendations also oppose counting a mosaic embryo automatically as having a disordered chromosome number; counselling specific to the person is needed.
Is a reported healthy birth a guarantee for me?
No. There are reports of healthy births; however, there is no proven system that reliably predicts the risks. The decision is made with counselling specific to you.
Is it compulsory to use an embryo with the chromosome number as expected first?
This is not a rule. Using such embryos first is regarded as a reasonable option left to your preference and assessed according to the person.
Is every centre's approach to mosaic embryos the same?
No. Policies on transfer, storing by freezing and fate may vary. You can learn your centre's approach before deciding.
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