If you’re familiar with preimplantation genetic testing for aneuploidy (PGT-A), you might have heard the terms “euploid” for genetically normal embryos and “aneuploid” for those with the incorrect number of chromosomes. Those with a mix of normal and abnormal cells are called “mosaic” embryos.
Deciding whether to transfer a mosaic embryo can be challenging, as it involves considering a complex mix of factors. To help you make an informed decision, we spoke to Dr. Robert Straub, a reproductive endocrinologist who’s been with Reproductive Biology Associates in Georgia since 2003. Dr. Straub specializes in ovulation induction and IVF, management of endometriosis and PCOS, endocrine disorders, and endoscopic surgery. Find out what he has to say about transferring mosaic embryos.
What is a mosaic embryo?
During PGT-A, DNA from a sample of cells taken from the embryo is analyzed under a microscope. This genetic testing helps IVF patients select embryos with the highest likelihood of resulting in a live birth before transfer.
Embryos are categorized based on their chromosomal status during PGT-A. The main categories include:
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Euploid — the correct number of chromosomes are present
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Aneuploid — contains the incorrect number of chromosomes
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Mosaic — there’s a mix of normal and abnormal cells
Dr. Straub explains, “A mosaic embryo is when some of the cells show normal DNA findings (euploid) whereas other cells show abnormal DNA findings (aneuploid).” The likelihood of a mosaic embryo resulting in a healthy baby depends on how many of those cells are abnormal.
“Mosaic embryos are further classified as low-level and high-level mosaic embryos,” he shares. RBA Fertility classifies mosaics based on the percentage of aneuploid cells present:
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Low-level mosaic (LLM) — approximately 20% of the DNA is found to be abnormal
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High-level mosaic — approximately 80% of the DNA is found to be abnormal
Do IVF clinics transfer mosaic embryos?
Many IVF clinics transfer mosaic embryos, though protocols may vary from one facility to the next. Talk to your reproductive endocrinologist to find out your options. Decisions are made on a case-by-case basis and will depend on the degree of mosaicism and your individual circumstances.
Dr. Straub shares that RBA Fertility currently performs embryo transfers with low mosaic embryos. “We have found that these types of embryos can result in a viable healthy pregnancy. However, the pregnancy rate with low mosaic embryos is reduced compared to normal (euploid) embryos.” RBA doesn’t transfer high mosaic embryos due to lower rates of viable pregnancies.
Can PGT-A results change over time?
You might wonder if a cryopreserved embryo can be retested later to produce different results. Dr. Straub says it’s possible to retest frozen embryos, but getting different results can cause confusion. “We do not typically retest embryos that have already been tested and have a clear definitive genetic result, even if a mosaic finding is identified,” he says. Because of the risks associated with thawing, re-biopsying, and refreezing an embryo, RBA only retests embryos if the initial results are inconclusive.
What are the risks of transferring a mosaic embryo?
Research shows mosaic embryos have lower implantation, pregnancy rates, and a higher chance of miscarriage compared to euploid embryos. Your doctor can determine your chances of a live birth based on your situation when transferring a mosaic embryo.
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Dr. Straub says pregnancies established with a mosaic embryo are monitored just like any other IVF pregnancy. Early on, patients will be monitored via serial hCG levels and ultrasounds to confirm proper growth and development of the pregnancy.
And just like other pregnancies, Dr. Straub encourages all women to continue pregnancy care with an OB/GYN and discuss noninvasive prenatal testing (NIPT) toward the end of the first trimester. “This noninvasive testing is yet another opportunity to screen and confirm the viability of the pregnancy,” he shares.
Making the best decision for your family
Although the decision to transfer a mosaic embryo shouldn’t be taken lightly, you don’t have to navigate it alone. Rely on your IVF team for support in making the best decision to help you achieve your family planning goals.
Your doctor will thoroughly review your genetic results, discussing the potential benefits and risks of embryo transfer. Dr. Straub emphasizes that this conversation will cover the value and possible drawbacks of using mosaic embryos. Additionally, patients are often referred to a genetic counselor to explore the implications, and a consultation with an embryologist can be requested for further information.
Alexa Davidson is a registered nurse and freelance health writer. She’s written for various women’s health companies, covering topics like natural hormone balance, fertility, and disease prevention. On her own fertility journey, Alexa has experienced profound loss and is passionate about supporting others with similar experiences. When she’s not researching or writing, Alexa can be found in the kitchen, where her specialty is making healthy versions of comfort foods. Nashville Hot Tofu, anyone?