PGD/PGS
Preimplantation Genetic Screening is a test performed by an embryologist in which one or two cells are removed from an embryo. The removed cells are then screened for genetic adnormallites.
Indications for PGD/PGS
1. Age over 35 years and have a higher risk of a baby with a chromosome problem.
2. Family history of chromosome problems.
3. History of recurrent miscarriage.
4. Recurrent IVF/ICSI failure.
Partner’s sperm is known to be at high risk of having chromosome problems.
Steps of PGD/PGS
1. You undergo normal IVF treatment to collect and fertilise your eggs.
2. The embryo is grown in the laboratory for two – three days until the cells have divided and the embryo consists of about eight cells.
3. A trained embryologist removes one or two of the cells (blastomeres) from the embryo.
4. The chromosomes are examined to see how many there are and whether they are normal.
5. One, two or three of the embryos without abnormal numbers of chromosomes are transferred to the womb so that they can develop. Any remaining unaffected embryos can be frozen for later use.
However, embryos that have been biopsied may not be suitable for cryopreservation and use in subsequent treatment cycles.
6. Those embryos that had abnormal chromosomes are allowed to perish or may be used for research (with you consent).
What stage should be done?
Actually we can do biopsy in
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Cleavage stage (D3)
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Blastocyst stage (D5)Day 3 embryo biopsy (6-8 cells) or day 5 embryos biopsy (blastocyst stage) both have advantage and disadvantage
Day 3 embryo biopsy will have more embryos to be chosen, but can only analyze 1-2 cell from each embryo. This testing cannot detect or rule out the possibility of abnormal chromosome, which have higher risk of mistaking.
Day 5 embryo biopsy, it is possible that instead of removing and testing one or two cells from a two-three day old embryo, some centers may allow the embryo to develop to five-six days, when there are 100-150 cells.
More cells can be removed at this stage without compromising the viability of the embryo, possibly leading to a more accurate test.
Alternatively some centers may test eggs for chromosomal abnormalities before they are used to create embryos. Polar bodies (small cells extruded by eggs as they mature) can be extracted and tested.
Mosaic embryos
PGD/PGS relies on the theory that all the cells in a human embryo are chromosomally identical, so that if you examine one cell from an embryo, it will show whether or not all the other cells have a chromosomal abnormality.
However, research has shown that in some embryos (known as mosaic embryos), the cells are not chromosomally identical. As a result, many such embryos will be discarded that are in fact capable of producing a normal pregnancy.
What are the risks of PGD/PGS?
Some of the risks involved in PGD/PGS treatment are similar to those for conventional IVF. Other problems unique to PGD/PGS treatment include:
- Some embryos may be damaged by the process of cell removal.
- Possibility that no embryos are suitable for transfer to the womb after PGS.
It is important that you appreciate that there is no guarantee against a miscarriage occurring even though PGD/PGS has been carried out prior to embryo transfer.
Recent studies have questioned whether or not PGD/PGS is effective at increasing the chance of having a live birth, there is a lack of evidence that having a treatment cycle with PGD/PGS will increase your chances of having a baby compared to having a treatment cycle without PGD/PGS