During this event, Dr Victoria Walker, Fertility Specialist at Institut Marquès, Spain, explored the complex topic of mosaic embryos and their implications for IVF success. Dr Walker explained how preimplantation genetic testing (PGT) categorises embryos as euploid, aneuploid, or mosaic. Mosaic embryos contain a mix of normal and abnormal cells, and their classification is crucial when deciding whether to transfer them.
Dr Walker emphasised that support, clarity, and personalised care are key when navigating mosaicism and implantation outcomes.
Mosaic embryos remain one of the most complex and emotionally challenging topics in IVF. Patients are often told they have embryos that are “neither fully normal nor fully abnormal,” leaving them uncertain about their real chances of pregnancy and a healthy baby. In this event, Dr Victoria Walker explained what mosaicism really means, how it is detected, why results can vary between laboratories, and how current clinical practice views the transfer of mosaic embryos. This article is based strictly on her presentation and reflects current clinical understanding.
Dr Walker began by reviewing early embryo development to provide essential context. After fertilisation, the embryo divides from one cell into two, then four, then eight cells. By day four, the embryo resembles a blackberry-like structure known as a morula. By day five, it becomes a blastocyst.
At the blastocyst stage, two distinct cell populations are visible. The inner cell mass will later develop into the fetus, while the outer layer, known as the trophoblast, will form the placenta. Importantly, all of this development occurs inside the egg’s outer shell until the embryo begins to hatch and prepare for implantation.
Dr Walker explained that embryo quality is traditionally assessed under the microscope, looking at factors such as cell number, symmetry, and fragmentation. While newer non-invasive tools and artificial intelligence are emerging, they are not yet routinely used in clinical practice.
At present, the main method for assessing chromosomal normality is PGT-A (preimplantation genetic testing for aneuploidy). This test does not assess the embryo itself directly but instead analyses a small sample of cells taken from the trophoblast.
In the laboratory, a small opening is created in the embryo’s outer layer using a laser, usually starting on day three. Over the following two days, some trophoblast cells herniate through this opening. Between four and ten of these cells are carefully removed for genetic analysis.
Because this number of cells is too small to analyse directly, they are first amplified and then analysed using next-generation sequencing. Results typically take two to three weeks and aim to determine whether the embryo has the correct number of chromosomes.
Dr Walker outlined four possible outcomes from PGT-A:
• Euploid: all tested cells are chromosomally normal
• Aneuploid: all tested cells are abnormal
• Mosaic: a mixture of normal and abnormal cells is detected
• Non-informative: the sample does not yield a reliable result and may require re-biopsy
The webinar focused on mosaic embryos, which occupy the grey area between clearly normal and clearly abnormal results.
Mosaicism occurs when some cells within the embryo are chromosomally normal while others are abnormal. Dr Walker emphasised that this result depends heavily on which trophoblast cells are sampled. Because the inner cell mass is not biopsied, mosaicism detected in the placenta-forming cells may not accurately reflect the genetics of the future fetus.
She illustrated how taking cells from different regions of the trophoblast could yield different results from the same embryo, ranging from euploid to mosaic or even fully aneuploid.
Dr Walker explained how next-generation sequencing results are interpreted. A normal embryo shows two copies of each chromosome. Aneuploid embryos show missing or extra chromosomes, such as trisomy or monosomy. Mosaic embryos may show partial gains or losses of chromosomes, indicating that only a proportion of cells carry the abnormality.
This complexity highlights why mosaicism is not always a clear-cut diagnosis and why interpretation requires experienced laboratories and clinicians.
One of the key points Dr Walker addressed was the wide variation in reported mosaic rates. Some laboratories report mosaicism in as few as 2–6% of embryos, while others report rates as high as 40%.
She explained that several factors may contribute to this variation:
laboratory techniques and culture conditions
the number of cells biopsied
the sensitivity of genetic analysis methods
operator skill during biopsy
possible contamination with maternal cells
the possibility that mosaicism is a normal developmental phenomenon
Dr Walker noted that some researchers believe many embryos may start as mosaic and self-correct during development, meaning mosaicism may be more common and less pathological than once thought.
Mosaic embryos can be classified in several ways to help guide clinical decision-making:
by the number of affected chromosomes (simple versus complex)
by the proportion of abnormal cells (low versus high mosaicism)
by whether whole chromosomes or only segments are affected
These categories are not absolute but help clinicians and patients weigh potential risks and benefits.
Dr Walker discussed reassuring data from published studies involving nearly 3,000 mosaic embryo transfers. The vast majority of babies born after mosaic embryo transfer were genetically normal. She explained that while mosaic embryos have lower implantation rates and higher miscarriage rates than euploid embryos, live birth is still possible.
If a mosaic embryo implants and progresses to birth, the chance of the child being chromosomally normal is very high, estimated at around 97–98%.
Dr Walker described how professional guidelines have changed over the past decade. Initially, mosaic embryos involving certain chromosomes, such as 13, 18, 21, X, or Y, were considered unsuitable for transfer. Patients were often advised to undergo another IVF cycle instead.
Current guidelines, including those from the European Society of Human Reproduction and Embryology, are more permissive. Low-risk mosaic embryos, particularly those with lower levels of mosaicism, may be considered for transfer after thorough counselling.
Clinics now commonly prioritise euploid embryos first, but do not automatically discard mosaic embryos.
All professional societies recommend genetic counselling both before and after PGT-A. Dr Walker stressed that patients must understand the uncertainties involved, including the possibility of miscarriage and the small but real risk of chromosomal abnormality.
Equally important is assessing how comfortable a patient feels with uncertainty. Some patients may prefer to proceed with a mosaic embryo rather than undergo further IVF cycles, while others may find the psychological burden too great.
Dr Walker concluded clearly that mosaic embryos do have a chance. They should not be automatically discarded, especially when no euploid embryos are available. While they are not transferred ahead of euploid embryos, they represent a meaningful option for many patients.
Based on clinical experience, evolving evidence, and reported live births, mosaic embryos can result in healthy pregnancies and healthy children when used thoughtfully and with appropriate counselling.
Whether to test at all depends on age and embryo numbers. Fertility Clinics Abroad’s guide to PGT-A notes that “PGT-A costs about €1,000-€3,000 for a batch of embryos at clinics in the Czech Republic, Greece, Poland, Denmark and Spain, against $4,000-$7,000 in the United States”, and that the HFEA rates the test red for most patients while the STAR trial found no gain per cycle.
It depends on the clinic. At Institut Marquès, we have what we call an EmbryoScope, which is a time-lapse technique incubator. The incubator takes photographs of the embryos every half an hour. So you get a little video of all your embryos, and you can log in and watch them evolving.
As your embryos are being made, you should be able to access those images. You can get little videos of that. You can also get photographs at the time of implantation, but usually it’s a small video on the day of the implantation.
We get the result from the lab. They translate it for us into English or French. We get the result, and if it’s very easy, we can just email it to the patient.
If you’ve got lots of normal embryos, great. If it’s all abnormal, then you get a phone call to go through that. If it’s a kind of mixture, then you might get a phone call as well. But yes, it’s all shared with the patient.
Difficult to say. It’s a complex one because it has three chromosomes, which is a negative factor, but it’s only affecting 30% of the cells, which is a low number. The embryo itself is of relatively good quality—it has a C in it. We will sometimes keep 4BCs if the embryo seems almost good. The likelihood of a live birth is very difficult to tell you.
I would give it a go. I don’t know your age or the other results. Is it the only embryo you have? I wouldn’t transfer this one first, but if it’s the only one you have, I would definitely think about doing it. Even if it’s a 5 or 10% chance of having a live birth, it might be worth it if that’s all you have.
Check everything to do with your uterus. Make sure you’re not missing anything there. Maybe you’ve done tests to look at the inside of the uterus. It’s not unusual for a euploid embryo to fail to implant, but you have to make sure the uterus is normal afterwards. If you can confirm the uterus is normal in all tests, I would give that embryo a go. I think I would give it a go.
Mosaics—clinics or labs decide where the cutoff is. Some embryos with high levels of mosaicism are categorised as aneuploid. Very low levels are considered euploid or normal. You’ve got this bandwidth in the middle, and that’s the hard part. Each clinic might use a slightly different bandwidth.
You’ve given it a really good go with seven cycles. That’s hard work. If you’re getting to the end, psychologically, it’s hard to leave an embryo behind if you think it might have a small chance of success.
It may not have a very good chance, especially since your euploid embryo didn’t implant. But if you’ve checked the uterus and done everything else, maybe try and use it. Then move forward with other treatment options, if you would consider them.
Yes, PGT can have an error rate. At our clinic, in the consent form, I think it’s between 2 and 5%. There is always the possibility that it’s not correct. But most of the time, we think it is.
Why do we think that? Because most studies that have been done on embryos that haven’t been transferred—abnormal embryos—are biopsied, and then more invasive tests are done to see whether the two correspond. Those are studies that have been done in the past, and most of the time, it’s felt that PGT-A is good, reliable, and in our clinic, we feel it’s a very useful tool.
It doesn’t necessarily give the results you want. It often tells you think you don’t want to hear, but psychologically, that can be very helpful in the longer term. We’re so used to seeing things written down—you see a report, it’s black on white, it tells you things, and you believe it. If you see in black and white that your embryo carries these trisomies or whatever, it somehow seems easier to accept rather than having miscarriages and not knowing what’s happening.
So, PGT-A, we feel, is very useful, even if it doesn’t give the results that you want. And yes, there’s an error rate. But if, for example, you fall pregnant with an embryo, you’re recommended to complete all the non-invasive tests to make sure the result you got for your embryo is true for your developing baby as well.
So yes, you need studies like the one you mentioned to make sure we’re doing it right and to ensure everybody is aware that it’s not 100%. Otherwise, you would go from transfer after transfer without knowing if it’s not working, and you wouldn’t know what’s going on.
Chromosome 9 is considered a low-risk chromosome, or used to be considered low-risk. It’s only one aneuploid, so it’s a simple abnormality.
It’s a slightly higher percentage, but it’s in that middle range where you don’t quite know whether it’s low or high. The chances of success are very difficult to know or give you accurately, but in my mind, because it’s a low-risk chromosome and it’s affecting under 50% of the cells, I would say you could try it. But I can’t give you a definite chance of success.
Yes, you can. Actually, in the presentation, I mentioned 3 methods of classification. There are others, and one of them is this situation, where you can have a euploid embryo and an abnormal trophectoderm on the biopsy, or the other way around: the embryo is normal, and the trophectoderm is not.
I don’t know if it’s one of the reasons euploids can miscarry, but it might be. You can have that. The more you learn, the more confused you get. You try to stick to the rules and think, “Ah, I’ve got it,” and then it slips away again. But yes, according to the classifications, you can have that.
This is getting harder and harder, I’m afraid. Segmental ones may be better than whole chromosomes. Two chromosomes are better than more than two or more than three. That’s probably low—it depends on which chromosomes are affected.
It’s very hard to give the live birth rate. Live birth rates when everything’s normal are probably around 50% per try, so it’s going to be somewhere under that. If you treat a segmental as a euploid, it depends on the chromosomes more than anything else. But then it would be about a 30–40% chance of success.
I think you should see a geneticist. I don’t have enough knowledge to answer that accurately. That’s exactly the kind of thing I would cut and paste and send to our geneticist, and ask them to see the patient. That patient has already been through a really tough life event. You want to give the patient a good chance of success, not another problem. I wouldn’t know what to say on this one—it’s more complicated. It’s better to have a consultation with the geneticist to give you the odds and the risks, and to understand the risk. That’s the hard bit. I think the risk makes sense.
Yes. So, it’s an interpretative test, but at the same time, from research, they feel most of the time that the biopsy they take is representative of the inner cell mass. There are those mosaics where they’re different, as I mentioned earlier, but most of the time in PGT, if you get a euploid result, research suggests the embryo is also euploid.
Mosaics can self-correct, yes. But PGT gives information. Obviously, you want to have a euploid embryo. If you get that, you feel confident using the embryo—it gives you confidence. If everything is aneuploid, that’s information you don’t want, but it’s still information. The mosaics are somewhere in between. One specialist in Australia said to me, “We just ignore mosaics,” because it’s like when we didn’t do PGT. That’s another position. You could say, “I’m not going to do PGT, because if I get mosaics, I’m still going to use them anyway.”
But I still think it gives useful information. If you get a euploid and an aneuploid result, that’s useful in itself. The mosaics—you might still use them—but you know they don’t have such a good chance of success. Then you have the information, and you know what is going on. I think it’s useful information.
I haven’t come across that. It’s not something I’ve talked about with anyone, but there are always certain cases—particular patients for one reason or another, in their particular life circumstances, religious reasons, or whatever. In our case, we would go to the directors and say, “There’s this patient, this is her situation, this is the reason, this is what she’d like to do. What do you think?” The directors often take it to clinical meetings, and the decisions are made. Can we help this patient to do what they want or not?
If it’s not going to be detrimental to the woman herself, then often we’ll be able to do things. Compassion is what we all try to be. I haven’t heard the term “compassionate transfer,” but you can understand why there might be reasons for transferring an embryo. At the same time, as another patient or commentator mentioned, we won’t transfer aneuploid embryos. If an embryo is completely abnormal, it doesn’t matter what people say; we won’t transfer it. We will keep them if patients don’t want to throw them away, but we won’t transfer them.
3AB is a lovely embryo. It’s not very expanded, but it’s a nice embryo. If it has complex abnormalities, that’s usually between 2 and 4 chromosomes, because if it’s more than 4, it might be chaotic. It depends on which chromosomes are affected; that might make you go one way or another. The percentage is under 50%, which is good—a lower percentage. Based on quality, you’d probably use that embryo if it hadn’t undergone PGT.
Looking just at morphological quality under the microscope, you’d probably use it. With the additional information of two to four chromosomes and less than 50%, I think in our clinic we would advise the patient to try it, understanding the embryo has a lower chance of implanting and a higher chance of miscarriage. But if it’s your only embryo left, then in that case, we probably would.
That’s a bit difficult because the clinic will have had a discussion with their lab about what cutoff they use to say this is aneuploid and this is mosaic. I don’t know how you would speak, you’d just explain you’d like more information. I can’t think of anything clever to say. Patients often want information, but sometimes clinics don’t have the information themselves.
They can have a conversation and say, “We don’t have this” or “We can’t give this.” For example, here in Spain, sex selection is illegal, as in most countries, so patients sometimes ask for information about their embryo and the answer is no. There’s no discussion.
You could go back to the directors and ask, but in this case, I think it’s a question of going to your particular doctor and explaining why you want the information. Ask if the lab can give you anything to help you understand your results better. Maybe ask to see a geneticist with the results. Even if you can’t see the result, maybe have an appointment with a geneticist and your doctor together so you all can better understand what you actually have.
This was a new egg donor. You must have stopped smoking, which may have affected his sperm and improved its quality. PGT-A makes money for the clinic, but you can get very different results from 2 different donors, even with the same sperm. It’s not necessarily the fact that you did PGT-A this time. It was probably a new donor, maybe better sperm if you stopped smoking. But it could just be a better cycle overall.
Just to give an example: I had a couple from Australia. From the first donor, we got no embryos at all. We used the same sperm sample with a second donor, and we had 1 to transfer and 3 to freeze. Exactly the same sperm sample, just a different cycle. Sometimes cycles don’t go well. Sometimes they get much better. I’d just be grateful for those 7 lovely blastocysts.
Age 33 embryos, I don’t know your history or how long you’d been trying at that stage. You were already doing IVF at age 33, so there was probably a problem that took you to the clinic that young, whether male or female factor. This is a day 7 embryo, from 6 years ago. It was unusual to freeze embryos on day 7, so your embryologist must have been confident about that embryo. They only keep the really beautiful ones on day 7. Statistically, at age 33, that embryo should still have a relatively good chance, not as high as a day 5 embryo, but still reasonable. In my mind, I’d go for the one from when you were 33 first.
Then see the geneticist and ask more about that deletion, because that’s an embryo from when you were older, and it needs more assessment. I can’t say necessarily with the mosaics what the success chance is. The purpose today was to see whether they should be used or not. But giving accurate chances of success is very difficult. I hope I haven’t misled anyone into thinking they should or shouldn’t be used.
You could have an aneuploid PGT result, but most clinics won’t allow transfer because they’re pretty sure of that result. I don’t know any clinics that would transfer an aneuploid embryo if it’s confirmed.
No. You can get embryos with multiple trisomies. That’s what you tend to see in older women. I’m assuming this is a mosaic embryo with multiple trisomies and monosomies. The more abnormalities you have, the lower the chance of success with that mosaic embryo. If you have five or six chromosomes affected, the embryo doesn’t have a good chance of success. You can still use it, but be aware of the lower chance. The embryo quality was good—5AB, if I remember correctly, so that would be an argument for using it. But with multiple abnormalities, understand that it’s not offering the best chance of success. As long as you’re aware.
Fertility Clinics Abroad reports that PGT-A costs about €1,000-€3,000 for a batch of embryos at clinics in the Czech Republic, Greece, Poland, Denmark and Spain, against $4,000-$7,000 in the United States. The IVF cycle and a frozen embryo transfer are extra.
