Mosaic Embryo: The High/Low Level Mosaic Embryo Evaluation

▲[EN SUB]Mosaic Embryo: The High/Low Level Mosaic Embryo Evaluation
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What is a Mosaic Embryo?

Mosaic Embryos are embryos that contain a mixture of normal and abnormal cells. If some cells in the embryo exhibit abnormalities in the chromosome analysis during Preimplantation Genetic Testing for Aneuploidy (PGT-A, formerly PGS), during IVF treatments, the embryo is referred to as a "Mosaic Embryo.
 

Embryo Mosaicism Level: How to Define High-level Mosaic Embryos and Low-level Mosaic Embryos?

The embryo mosaicism level refers to the proportion of abnormal cells found in the biopsy samples of cells.
 

Embryo Mosaicism Level	Proportion of Abnormal Cells
Normal embryo	≤ 20%
Low-level mosaic embryo	20-50%
High-level mosaic embryo	50-80%
Abnormal embryo	＞ 80%

How do you test for mosaic embryos?

Preimplantation Genetic Testing for Aneuploidy (PGT-A), formerly known as PGS, can be used to assess the mosaicism level in embryos. Embryos are microsurgeried during the PGT-A (PGS) process to obtain 5-10 trophoblast cells, which are then used for chromosomal analysis.

Lee Women’s Hospital PGT-A (PGS) equipment is the highest-resolution "high-resolution Next-Generation Sequencing analysis platform" (hr-NGS). When compared to traditional array Comparative Genomic Hybridization (aCGH), this platform provides a more accurate ability to screen embryos. In traditional array analysis, embryo mosaicism level is measured using a 50% cutoff. It can be further classified in today's high-resolution hr-NGS as normal embryos with mosaicism level less than 20%, embryos with low-level mosaicism ranging from 20-50%, embryos with high-level mosaicism ranging from 50-80%, and abnormal embryos with mosaicism level greater than 80%.

Should I transfer a mosaic embryo? How to prioritize mosaic embryo for transfer?

When choosing embryos for implantation, all three criteria must be considered at the same time.

• Embryo Grading (Appearance): Embryologists evaluate embryos and assign them a grade based on their appearance..
• Embryo Cleavage Rate: AI embryo selection systems can assess the rate of embryos cleavage.
• Embryo Mosaicism Level: Genetic testing can be used to assess mosaicism level

 

Case Study

A Couple A had two embryos ready for implantation, but wishes to only implant one. Which embryo should the doctor recommend for implantation in this scenario?
 

Criteria	Embryo 1	Embryo 2
Embryo grading（Appearance）	8	5
Embryo cleavage Rate	1	12
Embryo Mosaicism Level	Normal	Low-level mosaic

In this case, we would prioritize the implantation of the Low-level Mosaic Embryo, which has a success rate of more than 70%.

Pregnancy after Transfer of Mosaic Embryos

"Will implanted embryos with aneuploidy result in malformed babies?" we are frequently asked. "Does miscarriage become more likely?"

Our hospital has conducted extensive research on aneuploid embryos and discovered that even after implantation, these embryos have a chance of successfully implanting and developing into healthy babies.

Dr. Pin-Yao Lin at Lee Women’s Hospital has published research in the Journal of Clinical Medicine. According to the findings, while high-level aneuploid embryos have a higher miscarriage rate than low-level aneuploid embryos, the live birth rate is comparable.

Prenatal Testing After Transfer of Mosaic Embryos

When pregnancy occurs after the implantation of mosaic embryos, we are also concerned about the health of the fetus. As a result, we recommend chromosome testing for the fetus during pregnancy.

• Week 11 of pregnancy: Chorionic villus sampling
• Week 16 of pregnancy: Amniocentesis

Does chorionic villus sampling or amniocentesis have a risk of fetal deformity or miscarriage?

There have currently been no cases of fetal deformity or miscarriage as a result of sampling at Lee Women's Hospital. The accuracy rate of chorionic villus sampling can reach more than 80-90%, which is consistent with amniocentesis results.

In conclusion, the selection of implanted embryos needs to be evaluated based on the "3 criteria for embryo selection." In cases where there are fewer embryos available for IVF, the option of transferring mosaic embryos is worth considering. Doctors also advise scheduling chorionic villus sampling or amniocentesis after a mosaic embryo pregnancy to ensure the fetus's health.