What is Preimplantation genetic screening (PGS)?

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.

How and when is Preimplantation genetic screening (PGS) done?

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:

  • FISH-technique: Fluorescence in situ hybridisation (FISH) provides researchers with a way to locate and visualise a specific gene or DNA(Deoxyribonucleic acid is a molecule that encodes the genetic information of a human being) sequence using fluorescent probes. The fluorescent probes bind to specific areas on the chromosomes and detect any alterations that may cause a genetic condition.
  • PCR-technique: A PCR or polymerase chain reaction is a laboratory procedure in which millions of copies of a specific piece of DNA (Deoxyribonucleic acid is a molecule that encodes the genetic information of a human being) is made. PCR can be used in the analysis of mutations that occur in many genetic diseases (e.g. cystic fibrosis, sickle cell anaemia, phenylketonuria, muscular dystrophy).
  • aCGH-technique: Array Comparative Genomic Hybridisation (aCGH) is a method of genetic testing that allows the whole genome (all the genetic material present) to be analysed. It identifies minute alterations in chromosomes that would not be seen by a microscope.It can be used to quickly scan an entire genome for imbalances that may cause a genetic condition.
  • NGS: Next-generation sequencing applies to genome sequencing, and resequencing. It seems to be slightly more accurate that the aCGH.

In case you want to have more information on the different tests, please go to: http://www.igenomix.com/

Is it harmful for the embryo to do PGS?

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.

What are the reasons to do Preimplantation genetic screening (PGS) and who should use it?

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.

Does PGS include all genetic diseases?

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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