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Embryonic Mosaicism and IVF Embryo Screening

Embryonic mosaicism may be detected through IVF embryo screening, also known as preimplantation genetic screening or PGS. Before proceeding with IVF embryo screening, it is important to understand the complicated topic of embryonic mosaicism and how it is identified.



Embryo screening performed on embryos created through IVF is done for the purpose of providing information about the genetic health of the embryo. This information can be used to select the healthiest, most viable embryos for transfer.

Chromosomes are units that contain the genetic material of an embryo. Embryos with the usual 23 pairs of chromosomes – 46 in total – are the most likely to result in a successful pregnancy and live birth. Embryos that have too many or too few chromosomes can result in no pregnancy, early miscarriage, or occasionally a child being born with a genetic condition such as Down Syndrome.

The risk of embryonic aneuploidy (i.e., wrong number of chromosomes) increases with maternal age.



During the screening process, several cells are removed from the outer layer of the embryo for biopsy. In most cases, all of the cells are either normal or abnormal, and we can assume that the embryo itself is therefore chromosomally normal or abnormal. Sometimes, some of the biopsied cells appear normal, while others carry chromosomal abnormalities. This is known as embryonic mosaicism.

The challenge is in determining the percentage of abnormal cells that is indicative of mosaicism.

Since only a small number of cells are biopsied, it is possible that embryos labeled as mosaic could still develop into healthy babies. Laboratories often use different standards in determining which embryo biopsies are abnormal. A growing majority consider embryos with 20 to 80 percent abnormal cells to be mosaic.

Improved screening techniques can help increase screening accuracy. For instance, by delaying biopsy until day 5 or 6 (instead of day 3), more cells can safely be removed since extensive cell growth has taken place by that stage.



Although there are no clear guidelines, mosaic embryos that contain the largest percentage of normal cells and the lowest percentage of abnormal cells offer the best chance for pregnancy. When mosaic embryos are transferred, it may be recommended that those with less than 20 percent abnormal cells be prioritized.

Genetics counseling is advised for couples with mosaic embryos. Mosaic embryos can result in successful pregnancies, but problems with specific chromosomes do carry a greater risk of miscarriage or abnormal pregnancies.

Contact your local URA clinic in Hasbrouck Heights, Hoboken, and Wayne, NJ for more information. Your friendly URA team will be happy to provide you with more details about embryo mosaicism and provide a diagnosis.

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