Preimplantation genetic screening (PGS) is a test which is done to screen for abnormalities in the number of chromosomes in the embryos, obtained by In-Vitro-Fertilisation (IVF) or Intra-cytoplasmatic-sperm-injection (ICSI) prior to the transfer of the embryos into the uterus of the woman.
Normally an embryo has 46 chromosomes, 22 in pairs and additional the sex chromosomes, XX in female and XY in male. An embryo that does not have the correct number of chromosomes is called aneuploid. Changes in the numbers of the chromosomes can lead to no pregnancy at all, miscarriages, still births or disabled children.
The test has to be done on the cells from the embryo, therefore the biopsy procedure always involves two steps: the opening of the zona pellucida the outer shell of the embryo and the removal of the cell(s).
The biopsy will be performed on either day 3 or day 5. A 3 day old embryo has approximately 6 to 8 cells, and so only a single cell is removed for a biopsy done at day 3. A blastocyst, a 5 day old embryo has a few hundred cells, and so several cells can be safely removed during the biopsy.
The cells of the embryo will be tested with different methods:
In case you want to have more information on the different tests, please go to: http://www.igenomix.com/
If the embryo is handled correctly by a skillful embryologist, the embryo develops normally even after embryo biopsy. Published studies demonstrate that there is no increased rate of birth defects in IVF babies that are born after biopsy compared to IVF babies that are born without embryo biopsy.
Chromosomally normal embryos are the most likely to develop to term and to be born as a healthy baby.
Unfortunately, most of the time it is difficult to distinguish between a normal or abnormal embryo visually using a microscope. Hence, normal appearance of an embryo is not a reliable marker for it not having any genetic abnormality.
The risk of chromosomal abnormality is higher in women who are in their mod-thirties than those who are younger.
PGS is recommended in cases where the women has a history of recurrent miscarriage, recurrent implantation failure, who have had a prior pregnancy with a chromosome abnormality or where the man has abnormal sperm analysis. Any one or a combination of these factors, could increase the risk of abnormality in embryos.
No genetic test can detect all potential genetic abnormalities. PGS testing can only assess numerical changes in chromosome number and other imbalances in genetic material including deletions (part of a chromosome is missing) and duplications (part of the chromosome is double).
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