Mosaic Embryos: Should You Really Discard Them? | Dr. Marc Sklar

If you have been through IVF and your report came back with the word “mosaic,” you may have been told to set that embryo aside, move it to the bottom of the list, or discard it. In more than twenty years of working with women and couples navigating fertility, I have seen how painful that moment is. You count your embryos, you watch them develop, and then a lab report puts one of them in a category that sounds like a dead end. The science around mosaic embryos has moved forward a great deal in recent years, and much of what patients are told at their clinics has not kept up. Here is what PGT testing actually measures, what it misses, and what you can do to give your embryo the best possible environment.

Dr. Marc Sklar explains what a mosaic embryo really is and how to think about transferring one.

Why does a mosaic result feel like the end of the road?

When you invest so much in a cycle, a report that labels an embryo as anything other than “normal” can feel like a door closing. The truth is more hopeful and more complicated than a single word on a page. To make a good decision, you need to understand what the test is actually looking at, what mosaicism means biologically, and why your own body plays a much larger role than most patients are told. Let us walk through each piece.

What does PGT-A testing actually tell you, and what does it miss?

PGT-A stands for Preimplantation Genetic Testing for Aneuploidies. It checks whether an embryo appears to have the right number of chromosomes. A typical human embryo carries 46 chromosomes, 23 from each parent. When the report comes back, embryos are usually sorted into three groups. Euploid means the expected number of chromosomes. Aneuploid means an abnormal number, which is generally considered unlikely to result in a healthy pregnancy. Mosaic is the mixed category, and it is where I want to spend the most time.

Here is the part that surprises many patients. PGT-A is a biopsy. A small number of cells are removed from the outer layer of the embryo, called the trophectoderm, and tested. Those cells go on to form the placenta, not the baby. The inner cell mass, the part that becomes your child, is not directly sampled. So a PGT-A result reflects a small sample of cells that are destined to become supporting tissue. This is why false positives can happen, and why an embryo flagged as abnormal on the trophectoderm may still have a healthy inner cell mass. A PGT result is information, not a sentence. It deserves to be weighed alongside your full clinical picture, your history, and how well your body is prepared.

What you can do right now

• Ask your clinic for the full report, including the exact category for each embryo, not just a summary.
• Ask which cells were sampled and how many, so you understand the limits of the result.
• Keep the PGT result as one data point and pair it with your hormone, immune, and uterine health picture.

What is mosaicism, really?

The word “mosaic” sounds frightening, and it does not need to. A mosaic embryo is one where some cells carry a normal number of chromosomes and some do not. Picture a mosaic tile pattern, where some tiles are one color and some are another. It is not uniformly abnormal. It is mixed.

Does mosaicism automatically mean the embryo cannot become a healthy baby? Based on current evidence, no. Research published in journals such as Fertility and Sterility, along with data reported by preimplantation genetics groups, has shown that mosaic embryos can implant, can carry to term, and can produce chromosomally normal babies. There is even a described phenomenon sometimes called self correction, where abnormal cells appear to be selected out during development. Outcomes vary from case to case, so none of this is a guarantee.

The details matter a great deal. The degree of mosaicism is one factor. A low level mosaic embryo, often described as roughly 20 to 40 percent abnormal cells, is generally considered to have a more favorable outlook than a high level mosaic. The specific chromosome involved matters too. Some chromosomal patterns are associated with conditions that are compatible with life, and others are not. This is exactly the kind of nuance a good conversation with your provider should include.

One member comes to mind as I write this. She had two euploid transfers that failed, and she still had a mosaic embryo that her clinic was hesitant to use. We spent several months working on her uterine environment, her immune markers, and her inflammation before she transferred that embryo. She is now a mother. That is not luck. That is what can happen when you prepare the body rather than only classify the embryo.

What you can do right now

• Ask for the exact percentage of mosaicism, not just the label “mosaic.”
• Ask which chromosome is involved and what is known about it.
• Ask whether your embryo is considered low level or high level, and what that means for your situation.

Why do so many clinics default to discarding, and why is that outdated?

I want to be fair to reproductive endocrinologists. They work with the tools and protocols they have, and for a long time the guidance was cautious. If an embryo was mosaic and the risk was uncertain, the default was to avoid transfer. The science has since moved forward, and in many clinics the protocols have not caught up.

There is also a structural reason worth understanding. A clinic’s reported success is often measured as live births per transfer. If a clinic does not transfer a mosaic embryo, it does not risk lowering that number. That creates a real incentive to be conservative with mosaic transfers, even when a careful, individualized conversation might point in a different direction for you. I am not suggesting anyone is acting in bad faith. I am saying that if your clinic tells you a mosaic embryo simply cannot be transferred, you deserve a second opinion that includes the current literature, the degree of mosaicism, the chromosome involved, and what is happening in your own body.

On a recent group call, a member named Leena raised this exact issue. She had a frozen mosaic embryo from a previous cycle, a couple of options in front of her, and a clinic that was not giving her clear answers. Once we looked at her full picture, her hormones, her partner’s semen analysis, and her cycle health, she had a real roadmap instead of guesswork. That is what you need. Not just a category on a report, but a plan.

What you can do right now

• If you are told a mosaic embryo cannot be transferred, ask for the reasoning in writing and seek a second opinion.
• Ask how the clinic’s transfer policy is set and whether it is individualized to your case.
• Bring the current questions above to that conversation so it stays focused on your specifics.

Why is your body half of the equation?

Here is something that gets very little attention in the conventional IVF world. The uterine environment is not a passive container waiting for an embryo. Implantation is an immunological event. Your immune system has to recognize the embryo, tolerate it, and support it. When that process is disrupted by inflammation, autoimmune activity, gut issues, nutrient gaps, or thyroid dysfunction, even a genetically normal embryo can fail to implant.

Consider that euploid embryos fail too. Amanda came to us after three failed transfers using PGT tested euploid embryos. She was 40, she had two embryos left, and her clinic’s plan was a hysteroscopy followed by another transfer. There had been no real conversation about why three euploid embryos had not worked, and no investigation of her immune environment, her microbiome, or her thyroid. That gap is significant, and it is common.

When we prepare the body for a transfer, whether the embryo is mosaic or euploid, we look at markers such as inflammation, including CRP and related signals, thyroid function beyond TSH, including free T3, free T4, and antibodies, the uterine microbiome, immune activity, progesterone and luteal phase adequacy, and nutritional status, including CoQ10, vitamin D, and omega-3s. These pieces help determine whether an embryo has a real chance. The embryo is only half of the equation. Your body is the other half, and your body is something we can work with. Testing and any treatment should always be guided by your own provider.

What healthy preparation tends to include

• Inflammation that is well controlled rather than chronically elevated.
• Thyroid markers in a healthy range, looking beyond TSH alone.
• A balanced luteal phase with adequate progesterone support as advised by your provider.
• Nutrient status that supports egg and endometrial health.

When should you consider transferring a mosaic embryo?

Here is the practical guidance I know you are looking for. Every case is individual, so treat these as questions to bring to your provider rather than rules.

A mosaic embryo transfer may be worth serious discussion when it is your only or last embryo, because if discarding it leaves you with nothing, the balance of risk and benefit shifts and deserves a careful, data driven conversation. It may be more favorable when the mosaicism is low level, often described as under roughly 40 to 50 percent, so ask for the exact figure rather than the label. It matters whether the chromosome involved is one associated with survivable outcomes, since certain mosaic patterns have resulted in healthy births while others are far less likely to. It is stronger still when your uterine environment has been optimized, meaning your lining, immune balance, inflammation, and hormones have been addressed before any transfer. And it becomes especially relevant after a failed euploid transfer, because if chromosomally normal embryos have not worked, the issue may lie in the environment rather than the genetics, and that is where attention needs to go.

What you can do right now

• Make a short list of your embryos with the exact category and mosaicism percentage for each.
• Ask your provider which chromosome is involved and what the published outcomes look like.
• Prioritize preparing your uterine environment before scheduling any transfer.

How the HOPE Program approaches mosaic embryos

Inside the HOPE Program we use a three phase approach. We Assess, building a full root cause picture of what may be blocking pregnancy, including your PGT results but also your hormones, immune markers, gut health, and inflammation. We Personalize, building a protocol for your body and your specific situation rather than a generic plan. And we Support, walking with you through preparation, the transfer, the two week wait, and beyond. An embryo, mosaic or euploid, does not implant in a vacuum. It implants in your body, and your body’s environment matters enormously.

The bottom line on mosaic embryos

A mosaic embryo is not a failed embryo. Its potential depends largely on two things, the degree and type of mosaicism, and the body it is transferred into. The first you can understand by asking better questions at your clinic. The second is something you can actively build. The women who come to us after failed transfers, whether they had euploid or mosaic embryos, often share one thing in common. They were only ever given the genetics half of the picture, and no one had helped them optimize the other half, which is their own body. When you address both, things can change. Outcomes still vary from person to person, and your provider should guide every step.

Frequently asked questions

Is a mosaic embryo the same as an abnormal embryo?

No. A mosaic embryo contains a mix of cells, some with a normal chromosome count and some without, which is different from an aneuploid embryo where the sample shows a consistently abnormal count. Mosaic embryos can and do result in healthy pregnancies in some cases, though outcomes vary and should be discussed with your provider.

Can a mosaic embryo become a healthy baby?

Yes, it can. Published research and clinical experience show that mosaic embryos, particularly low level ones, can implant, carry to term, and produce chromosomally normal babies. The likelihood depends on the degree of mosaicism and the specific chromosome involved.

Does PGT-A test the actual baby?

Not directly. PGT-A samples cells from the trophectoderm, the part of the embryo that goes on to form the placenta. The inner cell mass, which becomes the baby, is not sampled, which is one reason results should be treated as information rather than a final verdict.

What percentage of mosaicism is considered low level?

Low level mosaicism is often described as roughly 20 to 40 percent abnormal cells, and it generally carries a more favorable outlook than high level mosaicism. Ask your clinic for the exact percentage on your report so you can have a specific conversation with your provider.

Why did my euploid embryo transfer fail?

A chromosomally normal embryo can still fail to implant when the uterine environment is not optimized. Inflammation, immune imbalance, thyroid issues, or luteal phase concerns can all play a role. Investigating these factors is often the missing step after a failed transfer.

Should I get a second opinion before discarding a mosaic embryo?

If your clinic tells you a mosaic embryo cannot be transferred and you have limited embryos, a second opinion that reviews the current literature, the degree of mosaicism, the chromosome involved, and your uterine health can help you make a fully informed decision.

About Dr. Marc Sklar, The Fertility Expert

Dr. Marc Sklar is a fertility specialist with more than twenty years of experience helping women and couples uncover the root causes behind their fertility challenges and build personalized plans to move forward. He is the founder of the HOPE Fertility Program and the creator of Fertility TV, where he shares evidence informed guidance with a global audience. Dr. Sklar trained in integrative and functional medicine and focuses on treating the whole person rather than a single test result.

Medical disclaimer: This article is for general educational purposes only and is not medical advice. It is not a substitute for diagnosis or treatment from a qualified healthcare professional. Always consult your own physician or fertility provider about your specific situation before making decisions about testing, embryo transfer, or treatment.

Connect with Dr. Sklar: YouTube · Instagram · Facebook · TikTok

References and further reading

• American Society for Reproductive Medicine (ASRM), guidance and patient resources on preimplantation genetic testing. asrm.org
• Preimplantation Genetic Diagnosis International Society (PGDIS), position statements on the management of mosaic embryos. pgdis.org
• Fertility and Sterility, peer reviewed research on mosaic embryo transfer outcomes. fertstert.org
• American College of Obstetricians and Gynecologists (ACOG), patient information on genetic testing and pregnancy. acog.org
• National Institutes of Health, MedlinePlus overview of chromosomes and mosaicism. medlineplus.gov