Course / What is Mosaicism & Why Should I Care?
Definition
Embryo mosaicism refers to the presence of two or more cell lines with different chromosomal complements within the same embryo.
For example, an embryo may contain both normal (euploid) and abnormal (aneuploid) cells.
Clinical Importance
Mosaicism directly affects embryo selection, reporting, and transfer strategy.
It may lead to misinterpretation if biopsy samples don’t represent the entire embryo.
Mosaic embryos challenge traditional “normal vs abnormal” classification models.
Impact in Surrogacy & IVF
In surrogacy, transferring a mosaic embryo raises specific ethical and counseling implications — since the genetic abnormality may or may not manifest in development.
Proper understanding allows agencies and clinics to ensure transparent decision-making among all parties.
Conceptual Model
Think of an embryo as a city made of cells.
In a euploid city, every “building” (cell) has the correct blueprint.
In a mosaic city, some buildings have a missing or extra floor — the plans vary.
When a small portion of cells (e.g., <30%) are abnormal, it’s called low-level mosaicism.
When a larger portion (e.g., 40–80%) are abnormal, it’s high-level mosaicism.
How It Happens
Mitotic errors during early cleavage divisions.
Environmental stress in culture or random chromosomal segregation events.
Unequal distribution of chromosomal material after fertilization.
Key Takeaway
The biopsy may sample only 5–10 cells from the trophectoderm (outer layer). These may not perfectly represent the inner cell mass that forms the fetus.
Hence, mosaic results are a probabilistic estimate, not an absolute diagnosis.
Clinical Transparency
In modern PGT-A practice, reputable laboratories must report mosaic findings according to recognized standards (e.g., PGDIS 2021 guidelines).
Clinicians should:
Review the full laboratory report and mosaic percentage.
Document the counseling process with intended parents.
Explain clinical implications, residual risks, and available choices.
For Surrogacy Programs
At Surrogacy4All or similar agencies, transparency is essential:
Mosaic findings should be disclosed to intended parents and, when appropriate, to the gestational carrier (with consent and within ethical bounds).
Embryo disposition and transfer strategy must follow informed consent procedures.
Incidence Data
Detected in 15–25% of blastocysts depending on lab technology and criteria.
Frequency increases with maternal age and culture duration.
False mosaic calls may arise due to technical noise in next-generation sequencing (NGS).
Lab-Dependent Variation
Some labs report <10% mosaicism, others >30%, depending on detection thresholds.
Validation of each lab’s mosaic classification system is critical before clinical interpretation.
Key Clinical Insight
Not all mosaic embryos are abnormal — many are self-correcting and can lead to healthy births.
Hence, the clinician’s experience and lab communication are key in interpreting results.
Stepwise Approach
When both euploid and mosaic embryos are available, transfer priority should follow this hierarchy:
Euploid (normal) embryos
Low-level mosaic embryos involving small chromosomes (e.g., chromosomes 21, 22)
Low-level mosaic embryos with whole-chromosome involvement
High-level mosaic embryos (case-by-case consideration)
Complex or segmental mosaics (transfer only after genetic consultation)
Key Considerations
Chromosome type: mosaicism involving critical chromosomes (13, 18, 21, X, Y) may carry higher clinical risk.
Percentage mosaicism: <30% considered low-level; >50% high-level.
Intended parents’ goals: willingness to accept uncertainty.
Surrogacy ethics: ensure full disclosure to all parties involved.
Recommended Protocol
Multidisciplinary review (fertility specialist + embryologist + genetic counselor).
Documented consent outlining potential outcomes.
Pre-transfer ultrasound and prenatal follow-up plan in place.
Clinical Decision-Making Framework
Confirm lab methodology — ensure NGS accuracy and thresholds.
Review embryo morphology and developmental grade — higher-quality mosaics have better implantation potential.
Assess type of mosaicism — segmental vs whole-chromosome.
Discuss outcome data — many published studies show live birth rates of 30–60% for low-level mosaics.
Establish informed consent with intended parents.
Surrogacy-Specific Caution
The gestational carrier’s role introduces added medical and legal responsibility.
Always involve a licensed genetic counselor and legal advisor prior to transfer approval.
Example Protocol
“At Surrogacy4All, mosaic embryo transfers are only considered after independent genetic review, embryology sign-off, and client consent documentation confirming awareness of clinical variability.”
Why It’s Crucial
Mosaicism sits at the intersection of science, probability, and ethics.
Genetic counselors provide nuanced explanations of residual risk, testing limitations, and pregnancy follow-up options.
They help intended parents make informed choices based on chromosome type, mosaic level, and personal values.
Topics Covered in Counseling
Mosaic classification terminology (low/high, segmental/whole)
Expected outcomes from published cohort studies
Prenatal diagnostic options (CVS, amniocentesis) after a mosaic transfer
Emotional and ethical guidance for decision-making
Best Practice:
Always include a formal genetic counseling step in any case involving mosaic findings — prior to embryo selection or transfer approval.
Partner with PGDIS-compliant laboratories that provide quantitative mosaic reporting.
Standardize your internal hierarchy policy for mosaic embryo transfer.
Maintain a central tracking database of mosaic outcomes for continuous improvement.
Prioritize blastocyst-stage biopsies — cleavage-stage biopsies increase mosaic detection errors.
Avoid discarding all mosaic embryos automatically — classify carefully.
Include genetic counselor involvement in both embryo reporting and consent discussions.
Implement clear documentation workflows within surrogacy programs to ensure legal and ethical transparency.
When in doubt, seek secondary lab review — mosaic categorization can vary.
Our job is to listen, to connect the dots between your needs, and to determine how we can best help you have your baby. If you’re asking how much does it cost for a surrogate, we’ll walk you through every step of the process to ensure there are no surprises.
To make an appointment with one of our counselors or physicians, please call (212) 661-7673 or email info@surrogacy4all.com. We look forward to hearing from you.
Surrogacy4All is a unique international surrogacy service targeted at providing state of the art IVF and surrogacy services to Intended Parents irrespective of age, sex or color, at the lowest possible cost in the world.
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