Understanding Mosaicism & the Power of PGT‑A: What Every Patient Should Know

Knowledge is powerful—but context is everything.

Preimplantation genetic testing for aneuploidy (PGT‑A) is one of the most transformative tools in modern fertility care. It helps us identify embryos with the correct number of chromosomes, reduce miscarriage risk, and select embryos with the highest chance of resulting in a healthy pregnancy.

But like any powerful tool, PGT‑A comes with complexity. And one of the biggest sources of confusion is mosaicism.

At Boston IVF, our goal is to bring clarity to the science and compassion to the decision-making. Because what you learn about your embryos can feel overwhelming.

You deserve explanations that empower—not alarm—you.

What Is Mosaicism?

In the simplest terms, a mosaic embryo contains a mixture of normal (euploid) and abnormal (aneuploid) cells.

A typical embryo is expected to be:

• Euploid: all cells chromosomally normal

OR

• Aneuploid: all cells chromosomally abnormal

But many embryos fall in between—a reality we only recognized once next‑generation sequencing became sensitive enough to detect it.

How mosaicism happens

Most aneuploidies (chromosome errors) arise during meiosis, when eggs or sperm are formed.

Mosaicism arises later, during mitosis, when the early embryo is rapidly dividing—meaning some daughter cells get the correct number of chromosomes while others don’t.

Different types of mosaicism

Mosaicism can occur:

• In the trophectoderm (cells that form the placenta) 
• In the inner cell mass (the part that forms the baby) 
• In both

This matters because PGT‑A tests only trophectoderm cells, not the inner cell mass—so the sample may not fully reflect the entire embryo.

Why Mosaicism Is So Complicated

PGT‑A is highly accurate when embryos are clearly euploid or aneuploid. But mosaic results are harder to interpret because:

• Not all mosaic embryos behave the same 
• Labs differ in how they classify mosaicism 
• Detection thresholds vary (3% in some labs, >20% in others) 
• It’s unclear whether some mosaic results reflect true mosaicism, technical noise, or sampling limitations 
• Some mosaic embryos self‑correct as they grow 
• Some mosaic embryos implant and result in healthy births

In fact, research now shows over 100 documented healthy live births from mosaic embryos. This is why mosaicism is not a simple “yes/no” result—it is a spectrum.

Can Mosaic Embryos Lead to Healthy Pregnancies?

Yes—many can. Studies show:

• Mosaic embryos implant less often than euploid embryos 
• They have higher miscarriage rates 
• But many still lead to healthy live births

Some mosaic embryos appear to “self‑correct,” meaning that as the embryo develops, healthier cell lines out‑compete abnormal ones. Sometimes abnormal cell lines end up confined to the placenta (confined placental mosaicism), not the fetus.

Large studies now provide clearer insight:

• Euploid embryos implant ~57% of the time 
• Mosaic embryos implant ~46% of the time 
• Miscarriage is more common (25% in some studies) 
• Low‑level (<50%) mosaics perform better than high‑level mosaics 
• Segmental mosaics often perform better than whole‑chromosome mosaics

Bottom line: Mosaic embryos are not all equal—and not all unsafe.

Segmental Mosaicism: A Special Category

A segmental aneuploidy means only a piece of a chromosome—not the entire chromosome—is duplicated or missing.

Some key points:

• Small segmental abnormalities sometimes self-correct 
• Some chromosomes tolerate segmental changes better than others 
• Large segmental abnormalities are more concerning 
• With genetic counseling, some patients choose to transfer segmental mosaics when no euploid embryos are available 
• These transfers can have the potential to result in healthy pregnancies

At Boston IVF, segmental mosaics are considered only after extensive genetic counseling, and always with informed consent.

Does Boston IVF Transfer Mosaic Embryos?

Our priority is always patient safety, ethical practice, and the best possible outcomes.

Boston IVF:

• Does not routinely classify embryos by mosaic levels (we work with labs to simplify reporting) 
• Prefers not to transfer mosaics when euploid embryos exist 
• Will consider transferring mosaic embryos only in specific cases:  
  
  • After all euploid options are exhausted 
  • With comprehensive genetic counseling 
  • After completion of a specialized consent process

For example:

A 43‑year‑old patient who has undergone multiple IVF cycles and has one mosaic embryo remaining may consider transfer with careful guidance.

How PGT‑A Helps—Even with Mosaicism

Despite these complexities, PGT‑A remains a powerful tool that can:

• Reduce miscarriage (50% of miscarriages are due to chromosomal abnormalities) 
• Improve embryo selection when multiple embryos are available 
• Reduce time to pregnancy 
• Lower the emotional and financial cost of failed cycles 
• Help identify embryos with major chromosome issues 
• Assist male factor infertility cases, where aneuploidy risk is higher

PGT‑A excels at identifying which embryos have the best chance—mosaic or not.

How Boston IVF Approaches Mosaic Results

We help patients understand:

• What the mosaic result means 
• How high or low the mosaic level likely is 
• Whether it involves whole chromosomes or small segments 
• What the data says about implantation and miscarriage 
• Whether self‑correction is possible 
• The risks, uncertainties, and ethical considerations 
• Whether transferring a mosaic embryo aligns with your family building goals

This is a shared decision-making process—never a directive. You are never alone in interpreting these results.

The Bottom Line: Mosaicism Doesn’t Mean Hopeless—It Means Informed

A mosaic result can feel confusing or frightening. But it does not mean:

✘ your embryo cannot implant 
✘ your embryo will become a child with genetic abnormalities 
✘ your journey is over

Instead, it means:

✔ your embryo exists on a spectrum 
✔ more information—and support—matters 
✔ many mosaic embryos do result in healthy births 
✔ you have options 
✔ our team will guide you every step

At Boston IVF, our mission is to ensure you understand the science and feel supported emotionally as you navigate your options.

Thinking About PGT‑A or Have Questions About Mosaicism?

Let’s talk—clearly, compassionately, and without pressure. Schedule a consultation with a Boston IVF physician or genetic counselor to explore:

• Whether PGT‑A is right for your IVF cycle 
• How to interpret your embryo results 
• What mosaicism means for your family-building plan 
• Your safest and most hopeful next steps

Your embryos are precious. Your decisions deserve clarity. Your future deserves hope.

And we are here—always—to guide you through it.