Preimplantation genetic diagnosis (PGD)

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What is preimplantation genetic diagnosis (pgd)

Embryo morphology is not accurate to detect the most viable embryos. One of the main reasons why good quality embryos fail to implant is because of their genetic status. The percentage of abnormal genetic embryos during IVF treatment is relatively high when patients are over 38 years (around 50-60%). Thus, PGS is used to detect chromosomal abnormalities and chromosomal rearrangements (translocations, inversions, deletions, etc). Embryos that have a normal number of chromosomes are more likely to result in a viable pregnancy. PGD is a technique, which enables couples with a particular inherited diseases in their family to avoid passing it on to their children. PGD is performed on embryo to determine if it has also inherited the genetic or chromosomal abnormality.

Preimplantation genetic testing of embryos is generally divided into:

Preimplantation genetic diagnosis (PGD)

One or more cells from an embryo are tested for the presence of a gene or genes that may harm the embryo and developing child, correct number of chromosomes, and translocation of a piece (or more) of chromosome from one to another chromosome.

Preimplantation genetic screening (PGS) is used to detect chromosomal abnormalities and chromosomal rearrangements (translocations, inversions, deletions, etc) and to test for the number of chromosomes. This is also known as aneuploidy screening.

For PGD is used the embryos which are created by assisted reproductive technology (ART). Up to two unaffected embryos are transferred to the woman’s uterus for implantation. If successful, the procedure will result in healthy pregnancy.

There are three types of biopsy such as polar body biopsy, blastomere biopsy and blastocyst biopsy (trophectoderm biopsy).

Polar Body Biopsy (Egg screening)

With this form of PGS the polar body is removed from the unfertilised egg. The polar body is not necessary for successful fertilisation or normal embryonic development and therefore this should have no deleterious effect on the embryo. The removed genetic material can then be analysed to determine whether there are any genetic abnormalities.

There are three types of biopsy such as:

Polar Body Biopsy (Egg screening)

With this form of PGS the polar body is removed from the unfertilised egg.  The polar body is not necessary for successful fertilisation or normal embryonic development and therefore this should have no deleterious effect on the embryo.  The removed genetic material can then be analysed to determine whether there are any genetic abnormalities.

Day 3 Biopsy (Blastomere Biopsy)

Blastomere biopsy is normally performed on the third day after fertilisation when the embryo has reached a 6-8 cell stage. In order to remove 1 or 2 cells containing a nucleus of the embryo, a hole has to be made in the zona pellucida. The advantage of sampling at this stage of development is that the genetic material of both parents can be examined. However, there is an increased chance of chromosomal mosaicism at this stage of development. This means that the cell that is removed and examined, may not be the same as the other cells of the embryo and a false result can be obtained. The result of the biopsy is known within a two weeks

Removing a cell from a day 3 embryo appears not to have a detrimental effect on the embryo development into a perfectly normal baby. It is not known at this time whether biopsied embryos have a lower chance of implantation than embryos without biopsy.

Day 5 Biopsy (Blastocyst Biopsy)

With a day 5 biopsy, several cells can be taken from the outer layer of the developing blastocyst (trophectoderm). These are the cells, which form the placenta. Studies have shown that this is not detrimental to the development of the embryo. By removing several cells, the accuracy of the test can be improved. One disadvantage of performing the test at this time is that the number of blastocysts available for sampling may be reduced in older patients or has connection with bad morphology of blastocysts. In addition, delaying the biopsy to this late stage of development limits the time to perform genetic studies. In this case, the blastocysts should be frozen and will be used in the next cycle.

An in vitro fertilization cycle with PGD involve:

  1. IVF procedure
  2. Biopsy
  3. Laboratory analysis
  4. Freezing/thawing of embryos
  5. Transfer

What are advantages of PGD?

It is an alternative to

  • prenatal diagnostic testing: Couples avoid the stress during pregnancy associated with diagnosis. 
  • termination of pregnancy: termination of pregnancy: when prenatal testing (amniocentesis or chorionic villus tests) reveals a genetic abnormality 

The opportunity to conceive a pregnancy that is biologically the parents’ own and yet unaffected by a genetic condition in the family

What are disadvantages of PGD?

The process of IVF and PGD poses some risks to the woman and the embryo:

  • Health risks of the IVF cycle An embryo may not develop after fertilization of an egg
  • Not all embryos will be suitable for biopsy
  • The embryo may not develop after biopsy
  • There may not be any unaffected embryos available for transfer
  • Test results may be inconclusive
  • Results are not 100% accurate
  • No pregnancy is achieved, even after the transfer of an unaffected embryo

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