Posted on January 18th, 2021by Dr. Erkan Buyukin Genetics

Mosaic Embryos: What does it mean, and how to handle it?

If you are undergoing fertility treatment using in vitro fertilization (IVF), it is highly likely that you had a conversation with your provider regarding testing the embryos before transferring them into your uterus. Called preimplantation genetic testing (PGT), this technology helps us identify the embryos that have the “correct” number of chromosomes (humans have 46 chromosomes, and this number changes from one specie to another). Following fertilization, the egg, now called a zygote, starts to divide and reaches a stage called “blastocyst” 5, 6, or 7 days later. An embryo at the blastocyst stage has two main parts that can be identified under the microscope: An inner part that will form the fetus and an outer part that will form the placenta. For the purpose of PGT, around 4-6 cells are taken from this outer part that will form the placenta and tested for the number of chromosomes. The embryo itself is frozen after the biopsy.

In theory, every cell of an organism has to have the same number of chromosomes. However, this is not always true, especially for embryos. Out of the 4-6 cells that are being tested, majority or all may be “normal” or “euploid”, indicating that they have 46 chromosomes; majority or all may be “abnormal” or “aneuploid”, indicating that they are lacking or they may have extra chromosomes. In the gray area between the two lies “mosaicism”. The threshold to detect mosaicism may change slightly from one lab to another, and if roughly 20-80% of the cells of an embryo are abnormal, then that embryo is called mosaic.

Mosaic embryos have also different types, depending on the percentage of the cells that are abnormal and depending on the type of abnormality. For example, if 20-40% of the cells that are tested are abnormal, then that embryo is “low-level” mosaic. If 40-80% of the cells are abnormal, then it is called “high-level” mosaic. Again, these thresholds may change from one laboratory to another. There are also different types of abnormalities. For example, some cells may miss a whole chromosome, more than one chromosome, part or parts of a single or multiple chromosomes; similarly, some cells may have an extra chromosome, extra multiple chromosomes or extra parts of chromosome(s).

Although the transfer of mosaic embryos can at times lead to healthy pregnancies and babies, the decision to transfer a mosaic embryo is a complex one that needs to be discussed with one’s provider and a genetic counselor. Currently, we do not know the long-term outcome of the babies born following these transfers. If there are normal embryos available, then the priority is always to transfer the normal embryo first. If there is not a normal embryo available, then we encourage patients to undergo another IVF cycle in order to have a normal embryo for transfer. If all options are exhausted and there is no other embryo available for transfer, then the transfer of a mosaic embryo can be considered following a thorough counseling, ideally with a genetic counselor.

In vitro fertilization is a sophisticated treatment with a lengthy journey to parenthood. Stumbling upon mosaic embryos during this journey is just another piece of the puzzle that needs to be resolved. Here at RMA of New York, we have a dedicated team of providers and genetic counselor who will help you understand what mosaicism is and help you to make an informed decision.

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