IVF in the United States: Are Chimeric Embryos Suitable for Transfer?

Pre implantation genetic testing has been widely used in the process of in vitro fertilization, significantly increasing the chances of successful pregnancy. However, in testing, multiple issues may sometimes be found with embryos, one of which is chimeric embryos. Whether these embryos are suitable for transplantation and the possible situations that may occur after transplantation are issues that need attention in IVF.

What is a Chimeric Embryo?

Chimeric embryo refers to the presence of two or more cells with different chromosomes in the constituent cells of an embryo. Chimeric embryos can be divided into homologous chimeras and heterologous chimeras. Homologous chimeras refer to cell lines that come from the same body but have different genetic properties. Usually, due to abnormal chromosome numbers in sperm or eggs or abnormal division of fertilized eggs, early division errors occur during embryonic development, resulting in the formation of two or more different cell lines that continue to divide and form their own daughter cells, thus forming homologous chimeras. Heterozygous chimerism usually occurs during amniocentesis, which means that different fertilized eggs merge and develop into one cell, or the chromosomes of two fertilized eggs fuse with each other. In rare cases, it occurs naturally, while in most cases, heterologous chimeras are artificially merged in the laboratory.

Normally, all cells in an embryo are produced by the division of a single cell. If errors occur during cell division, it may lead to the formation of chimeric embryos. This may cause various problems, such as heart disease, deformities, intellectual disability, autism, etc., and there is no specific treatment method, so the only option is to terminate the pregnancy, which also increases the risk of miscarriage.

Is chimeric embryo suitable for transplantation?

Before deciding whether it is suitable for transplantation, the American Little Monster IVF expert first explained the priority classification of chimeric embryo transfer:

Level 1: Embryos with completely normal chromosomes are suitable for transplantation, while non diploid embryos should never be transplanted. Level 2: Embryos with a chimerism ratio of no more than 20% can be transferred, while embryos with a chimerism ratio of more than 80% cannot be transferred. Level 3: If other embryos cannot be obtained, haploid chimeric embryos with a chimerism ratio between 20% and 80% can be considered for transplantation, but Turner chimerism cannot be transplanted. Level 4: If other embryos cannot be obtained, consideration can be given to transferring triploid chimeric embryos of chromosomes 1, 3, 4, 5, 6, 8, 9, 10, 11, 12, 17, 19, 20, 22, X or Y with a chimeric ratio between 20% and 80%. Level 5: If other embryos cannot be obtained, consider transferring triploid chimeric embryos of chromosomes 14 and 15 with a chimeric ratio between 20% and 80%. Level 6: If other embryos cannot be obtained, consider transferring triploid chimeric embryos of chromosomes 2, 7, and 16 with a chimeric ratio between 20% and 80%. Level 7: If other embryos cannot be obtained, consideration can be given to transferring triploid chimeric embryos of chromosomes 13, 18, and 21 with a chimeric ratio between 20% and 80%.

In summary, if it is possible to obtain other embryos, do not rush to transfer them. Instead, preserve the chimeric embryos that meet the transfer criteria. If there are indeed no other available embryos in the future, then consider whether to transplant them. For cases where other embryos cannot be obtained, whether they can be transplanted depends on whether they meet the criteria for transplantability, while considering other factors such as testing methods, degree of chimerism, which chromosome the chimerism occurs on, and the needs of the fertilized individual. The specific situation may vary.

Third generation IVF technology ensures the health of embryos

On the fifth day of in vitro culture, when entering the blastocyst stage, the fertilized egg has already divided into more than 100 cells, and the vitality of the embryo is even stronger. At this point, the test tube experts from Little Monster in the United States will select some cells from the blastocyst for chromosome and genetic testing. Embryo chromosome screening is mainly used to detect abnormalities in chromosomes, such as chromosome duplication, deletion, translocation, and inversion, which may lead to Down syndrome, Edwards syndrome, intellectual disability, and autism