Preimplantation Genetic Screening is a test performed by an embryologist in which one or two cells are removed from an embryo and then screened for genetic abnormalities.
When PGD/PGS Is Recommended
- People older than 35 years and have a higher risk of a baby with a chromosome problem.
- A family history of chromosome problems.
- History of recurrent miscarriage.
- Recurrent IVF/ICSI failure.
- Partner’s sperm is known to be at high risk of having chromosome problems.
- You undergo normal IVF treatment to collect and fertilize your eggs.
- The embryo is grown in the laboratory for two – three days until the cells have divided and the embryo consists of about eight cells.
- A trained embryologist removes one or two of the cells (blastomeres) from the embryo.
- The chromosomes are examined to see how many there are and whether they are normal.
- 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.
- Those embryos that had abnormal chromosomes are allowed to perish or may be used for research with your consent.
When can the PGD process take place?
Actually, we can do a biopsy in the Cleavage stage (D3) and Blastocyst stage (D5)
Day 3 embryo biopsy (6-8 cells) or day 5 embryos biopsy (blastocyst stage) both have their own advantages and disadvantages
Day 3 embryo biopsy will have more embryos to choose from, but only has 1-2 cell from each embryo that can be analyzed. This testing cannot detect or rule out the possibility of an abnormal chromosome.
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.
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:
1. Some embryos may be damaged by the process of cell removal.
2. 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.
PGD/PGS will increase your chances of having a healthy baby compared to having a treatment cycle without PGD/PGS.