During this live Q&A session, we explored the complex and evolving topic of mosaic embryos — what they are, how they are identified through PGT-A (preimplantation genetic testing), and why they matter in IVF treatment planning. Mosaic embryos contain a mix of chromosomally normal and abnormal cells. This raises important questions for both patients and fertility specialists: Are these embryos viable? Should they be transferred? And what are the chances of a healthy pregnancy?
Featuring Experts:
Hosted by: Sandy Christiansen, MSc, Clinical Embryologist, Fertility Educator, and Advocate.
Our expert guests shared insights into the science behind mosaicism, the accuracy and limitations of current testing methods, and how different clinics interpret mosaic results. They also addressed how mosaic findings can influence clinical decisions and emotional experiences for patients.
César Arjona Fernández, Genetic Counselor, Fertilab, Spain: I am a genetic counselor at Fertilab, a fertility clinic in Barcelona. My role focuses on providing genetic consultations for patients who require genetic testing during their fertility treatment and supporting them with information throughout that process.
PGT is a technique used to diagnose embryos by characterising their chromosomal composition. It is offered to couples who have an increased risk of producing embryos with chromosomal abnormalities. One of the main indications is maternal age, as many people now begin their reproductive journey later in life. Other indications include recurrent miscarriage, structural chromosomal abnormalities in one of the partners, or other known genetic risks. In recent years, PGTA has also been chosen by some patients to increase the chance of pregnancy per transfer, even without a specific medical indication. These are the most common reasons for offering PGTA to patients.
Rostislav Navratil, PhD, Head of Molecular Genetics Lab, Repromeda, Czechia: I am a molecular geneticist and Head of the Molecular Genetics Laboratory at Repromeda in the Czech Republic. I have worked in this field for nearly 11 years, specialising in preconception genetic testing and preimplantation genetic testing of embryos, including mosaicism.
One common result is full aneuploidy, meaning that an entire chromosome is missing or present in an extra copy in all cells of the biopsy. Most of these embryos are not viable. However, certain chromosomes, such as chromosome 21, 18, or 13, are associated with conditions like Down syndrome and can sometimes result in viable pregnancies.
Another type of result is segmental aneuploidy, where only part of a chromosome is missing or duplicated. Both full and segmental aneuploidies can also occur in mosaic form. In mosaic embryos, the biopsy contains a mixture of normal (euploid) cells and abnormal (aneuploid) cells. Biologically and genetically, the abnormality is the same, but the difference lies in the proportion of affected cells.
Rostislav Navratil, PhD, Head of Molecular Genetics Lab, Repromeda, Czechia: Mosaicism refers to a situation where an organism consists of two or more cell lines with different genetic information. In IVF embryos, this means a mixture of euploid and aneuploid cells. During a biopsy, typically five to ten cells are removed. If only some of those cells are aneuploid, the result may be reported as mosaic, for example, 30% mosaicism.
Cutoff values vary between laboratories. In our laboratory, mosaicism is reported between 30 and 80%, with lower values classified as low-level mosaicism and higher values as full aneuploidy. Professional societies, such as the European Society of Human Reproduction and Embryology, recommend a lower cutoff of around 20%. These thresholds differ between providers.
César Arjona Fernández, Genetic Counselor, Fertilab, Spain: There is no legal regulation defining a universal cutoff. Each genetic testing provider has its own internal controls and statistical analysis systems. Even if procedures are similar, they are not identical. This makes it difficult to standardise mosaicism definitions across laboratories. When working with a specific genetic provider, clinics must follow that provider’s reporting rules and manage patient care based on those results.
César Arjona Fernández, Genetic Counselor, Fertilab, Spain: In practice, patients are often not well prepared. It is essential to counsel patients thoroughly before testing so they understand all possible outcomes. Many patients expect only euploid or aneuploid results. However, mosaic results occur in approximately five to 15% of PGT-A cases.
If patients are not informed in advance, they may feel confused or distressed when they receive mosaic results and may not understand what those results mean for pregnancy potential or health outcomes. Managing expectations before testing is critical.
Rostislav Navratil, PhD, Head of Molecular Genetics Lab, Repromeda, Czechia: Embryologists usually perform the embryo biopsy but not the genetic analysis itself. The biopsy is sent to a molecular genetics laboratory, where PGT-A is performed. The basics of PGT-A are straightforward, but performing it at a high level requires years of experience, continuous education, and familiarity with professional guidelines and current research. Even after 11 years in the field, learning continues.
The procedure involves many critical steps: embryo biopsy, transferring the cells into tubes, DNA amplification, and sequencing. Patients often focus only on the biopsy, but each step is equally important for obtaining reliable results.
César Arjona Fernández, Genetic Counselor, Fertilab, Spain: In experienced laboratories, PGT-A is safe for good-quality embryos. If an embryo stops developing after biopsy, it is usually due to poor embryo quality rather than the biopsy itself. In good hands, PGT-A is not harmful to high-quality embryos.
Rostislav Navratil, PhD, Head of Molecular Genetics Lab, Repromeda, Czechia: Current vitrification methods are highly effective and do not affect embryo quality. PGTA biopsy is usually performed before freezing. In some cases, frozen embryos may be thawed, biopsied, and refrozen, such as when a rebiopsy is recommended for certain segmental findings. Success depends primarily on embryo quality rather than the freezing process itself.
César Arjona Fernández, Genetic Counselor, Fertilab, Spain: Some patients accumulate embryos over several cycles before PGTA. Performing a biopsy on previously frozen embryos is not uncommon. The main concern is embryo quality, not whether the embryo has been frozen.
Rostislav Navratil, PhD, Head of Molecular Genetics Lab, Repromeda, Czechia: The difference is related to batching and cost. PGT-A uses next-generation sequencing, which is expensive. Laboratories typically analyse embryos in large batches to maximise machine capacity. This requires time to collect enough samples. Faster turnaround times are possible, but cost more because fewer samples are processed at once.
César Arjona Fernández, Genetic Counselor, Fertilab, Spain: In our clinic, results are explained by embryologists or genetic counselors. Some genetic laboratories also offer direct genetic counseling services, but clinic-based counseling is most common.
Rostislav Navratil, PhD, Head of Molecular Genetics Lab, Repromeda, Czechia: In the Czech Republic, results are always provided to the physician, who discusses them with the patient. For mosaic or complex findings, consultation with a clinical geneticist is recommended.
César Arjona Fernández, Genetic Counselor, Fertilab, Spain: Low-level mosaicism does not appear to be strongly related to age. High-level mosaicism may be more common with increasing maternal age, but this understanding continues to evolve.
Rostislav Navratil, PhD, Head of Molecular Genetics Lab, Repromeda, Czechia: Mosaicism is not affected by age in the same way as full aneuploidy. Older women may show slightly higher mosaic rates because they produce more aneuploid embryos, some of which may partially self-correct through mechanisms such as trisomy rescue.
César Arjona Fernández, Genetic Counselor, Fertilab, Spain: Current methods are reliable for detecting euploid and aneuploid embryos. Mosaic embryos, however, are complex. Any mosaic embryo has the potential to result in a healthy pregnancy, although with a higher risk and lower implantation potential compared to euploid embryos. Patients must understand and balance these risks through appropriate genetic counselling before deciding on transfer.
Rostislav Navratil, PhD, Head of Molecular Genetics Lab, Repromeda, Czechia: The most important factor is experience. Clinics and laboratories that perform PGT-A on thousands of embryos each year usually have more robust processes. That said, newer laboratories can also be very good, so this is not an absolute rule.
If I were a patient, I would prioritise experience, followed by the specific procedures and technologies used. There is no single universal PGT-A test. Different laboratories use different methods, software, and additional analyses to improve accuracy. For example, our laboratory develops its own PGT-A tests and adds complementary techniques to increase reliability.
Accuracy is a cornerstone of good PGT-A. Poor-quality embryos can generate poor-quality data, which increases the risk of false-positive results, such as calling an embryo mosaic when it is actually euploid, or false-negative results, where a full aneuploid embryo is misclassified. We use additional tools, such as single-nucleotide polymorphism analysis, to help distinguish true mosaicism from technical artefacts.
Much of this information can be found on laboratory websites, but it is often difficult for patients to access or interpret.
César Arjona Fernández, Genetic Counselor, Fertilab, Spain: From the patient’s perspective, this information is very difficult to obtain. Patients usually choose an IVF clinic, but the clinic decides internally which genetic laboratory performs the testing. These are 2 separate worlds for patients.
Most patients rely on clinic reputation, patient forums, and word of mouth rather than technical details about genetic laboratories. If an IVF clinic has a strong reputation, it is likely working with a reliable genetic provider. However, it is currently very difficult for patients to choose a clinic specifically based on the genetic laboratory used.
César Arjona Fernández, Genetic Counselor, Fertilab, Spain: Chromosomal constitution of the embryo is crucial for implantation, but it is not the only factor. If it were, every euploid embryo transfer would result in pregnancy, which we know is not the case. Even with euploid embryos, implantation rates are around 60–80%, depending on the clinic.
Mosaic embryos should be offered to patients, provided they are fully informed. Current evidence shows that mosaic embryos, particularly those with a low level of mosaicism, can result in healthy pregnancies and healthy babies. Outcomes for low-level mosaic embryos, generally below 50%, are quite similar to those of euploid embryos. High-level mosaic embryos have poorer outcomes.
What is essential is transparent counseling. Patients should receive accurate information about what is known today, without exaggerating risks or withholding options. Based on this information, and respecting patient autonomy, couples can decide whether to proceed with transfer.
Rostislav Navratil, PhD, Head of Molecular Genetics Lab, Repromeda, Czechia: In this specific case, there is no reason to avoid mosaic embryos in the future. The miscarriage was very unlikely to be caused by mosaicism, as the fetus was genetically normal.
If a patient has both euploid and mosaic embryos, the euploid embryo should always be transferred first. If that does not result in pregnancy, transferring a mosaic embryo is a reasonable option, as there is still a significant chance of achieving a healthy pregnancy.
Rostislav Navratil, PhD, Head of Molecular Genetics Lab, Repromeda, Czechia: There is no universal standard. Each laboratory has its own decision tree for how mosaicism is defined and reported. In our laboratory, we define mosaicism between 30% and 80% and clearly state the percentage in the report.
Terms such as LLM or HLM can mean different things in different laboratories. Without clear definitions in the protocol, these labels are not helpful. This information must be transparent, and genetic counseling is essential to ensure patients understand what the results mean.
Some clinicians choose not to receive or disclose detailed mosaic information because they feel uncomfortable managing the associated risks. In my opinion, this is poor medical practice. If embryos have potential value, patients should receive complete information and be allowed to decide how to proceed.
César Arjona Fernández, Genetic Counselor, Fertilab, Spain: Patients may encounter two situations: either the clinic has the detailed information but chooses not to present it, or the clinic chooses not to receive that information at all. Both situations occur.
The principle of patient autonomy requires that patients receive full and accurate information so they can make informed decisions. Simplifying results may feel easier, but it can limit patient choice.
César Arjona Fernández, Genetic Counselor, Fertilab, Spain: We now have many parameters to assess embryos: morphology, morphokinetics, PGT-A results, mitochondrial activity, transcriptomic data, and even maternal endometrial factors. Managing all this information together is extremely complex.
Currently, morphology and chromosomal constitution remain the most reliable indicators of implantation potential. Transcriptomic and mitochondrial data are promising but are not yet reliable enough to guide transfer decisions beyond what we already use. These parameters may complement decision-making in the future, but at present, they should not outweigh established criteria.
Rostislav Navratil, PhD, Head of Molecular Genetics Lab, Repromeda, Czechia: Transcriptomic analysis is very promising, but it is still at an early stage. We have seen similar enthusiasm before, for example, with non-invasive PGT-A from culture media, which ultimately did not deliver reliable results.
PGT-A based on biopsy is currently the best available method. Transcriptomics and other molecular approaches may improve embryo assessment in the future, but we are not there yet.
Rostislav Navratil, PhD, Head of Molecular Genetics Lab, Repromeda, Czechia: PGT-A interpretation involves human judgment, and different specialists can reach different conclusions when evaluating the same data. AI has the potential to reduce this subjectivity by learning from vast numbers of PGT-A profiles.
AI could eventually outperform humans by analysing patterns across thousands or millions of cases. I see great potential for AI in PGT-A, especially for standardising interpretation and reducing bias.
César Arjona Fernández, Genetic Counselor, Fertilab, Spain: AI is already used in embryology to assess embryo development and implantation potential. However, relying on a single tool is not sufficient. The best approach is to integrate multiple tools: morphology, time-lapse data, PGT-A, and AI-based analysis.
Using these tools together may improve outcomes, but AI alone cannot replace clinical judgment at this stage.
Rostislav Navratil, PhD, Head of Molecular Genetics Lab, Repromeda, Czechia: Possibly in the future, but we are not there yet. Current non-invasive approaches do not have sufficient reliability. That said, the pace of AI development over the last few years has been remarkable, and it is possible that this will change over the next 5 to 10 years.
César Arjona Fernández, Genetic Counselor, Fertilab, Spain: At present, biopsy-based PGT-A remains the most reliable option. AI and non-invasive methods are promising, but they are not ready to replace current practice.
César Arjona Fernández, Genetic Counselor, Fertilab, Spain: Embryologists continuously monitor embryo development, assessing division timing, fragmentation, and developmental dynamics. These parameters are used to grade embryo quality and determine suitability for biopsy.
The decision to biopsy is based on a combination of these observations. There is no single grading threshold; it is a holistic assessment made by the embryologist at the time of biopsy.
Rostislav Navratil, PhD, Head of Molecular Genetics Lab, Repromeda, Czechia: Information from the embryology laboratory is crucial for interpreting PGT-A results. Poor embryo quality can result in noisy genetic data that reflects technical issues rather than true biology.
Details such as abnormal cell division, embryo degeneration before biopsy, or cell lysis during sampling are extremely important for us. These embryos may still implant, but they generally have lower implantation rates. Close communication between embryologists and genetic laboratories is essential.
