Preimplantation Genetic Diagnosis Process
Preimplantation Genetic Diagnosis (PGD) for genetic disorders
Every couple planning a family hopes for a healthy baby.
But for some couples, the risk of having a baby with a genetic disorder such as cystic fibrosis, Down syndrome or Huntington disease is much higher.
If you and your partner are concerned about your baby being born with a genetic disorder, there are screening options available even before getting pregnant.
PGD can be used to screen your embryos allowing those embryos that contain an obvious genetic abnormality to be excluded. Testing can involve a count of the chromosomes and/or a molecular examination for a particular gene or mutation. Either way, the testing can increase both the chance of a genetically normal pregnancy and your chance of having a healthy baby.
Genea is one of the very few centres in Australia with the vital combination of IVF and genetics facilities to perform these sophisticated tests successfully.
Getting started with PGD
If you wish to make an appointment to find out more about PGD or whether this treatment is right for you, please follow the steps below:
- Obtain a referral from your GP to one of our Clinical Geneticists or accredited Fertility Specialists who will be responsible for your care.
- Make an appointment with a Clinical Geneticist or Fertility Specialist by calling their rooms.
What happens in PGD?
You will need to have an IVF cycle to create embryos, just like couples with infertility undergoing an IVF treatment.
PGD requires the biopsy or removal of cells from each embryo for analysis.
At Genea, our advanced embryo culture techniques allow us to wait until the embryos have reached the optimum fifth day of development when they can have a hundred or more cells, and then remove three to four at a time. Other clinics conduct the biopsy at Day 3 of the embryos' development when they consist of just six to eight cells, and only a single cell is removed, greatly reducing the number of opportunities for success.
By waiting until embryos have reached the blastocyst stage, Genea scientists can select cells from the trophectoderm, the part of the embryo that will go on to form the placenta. The inner cell mass, the part that will become the baby, is not touched.
In the diagrams at the end of this page, you can see the process illustrated.
Genea scientists can use a number of different methods to analyse the biopsied cells.
For chromosome analysis, Genea uses a technique called comparative genome hybridisation or CGH to analyse biopsied cells.
CGH has many advantages over the outdated FISH technique - the main one being that it can accurately assess ALL of an embryo's chromosomes, rather than 9 that FISH allowed.
To begin, the biopsied cells are subjected to whole genome amplification (WGA), a process that results in the production of multiple copies of the DNA being investigated.
The CGH techniques then requires an extensive analysis procedure of laboratory time to reveal the chromosome status of the embryo and it is therefore a requirement that all embryos undergoing CGH analysis be vitrified (stored in liquid nitrogen) whilst the analysis is conducted. Genea have been vitrifying day 5 blastocysts clinically since January 2006.
Embryos found to have a normal chromosome complement can then be transferred in subsequent frozen embryo cycles.
If the problem is at a gene level rather than chromosome it is more common to use PCR.
PCR makes millions of copies of a part of the DNA code, which allows us to see whether this part of the DNA in the sample is normal or mutant.
Top: Fluorescently labelled DNA from the cells biopsied from a human embryo. Our cytogeneticists capture 3 fluorescent images and overlay them to produce the result.
Right: The analysis data generated by our cytogeneticists and computer system shows us all the embryo's chromosomes. The close up shows a trisomy of chromosome 18.
Even in the blastocyst, the embryo's cells are not in the final form they will have as a fetus and placenta after implantation. PGD for chromosome counting (preimplantation genetic 'screening' rather than true 'diagnosis') is a testing procedure that reduces the chance of Down syndrome or having a miscarriage, for example. But it does not eliminate these possibilities. So even if you have PGD, you should still have the usual first trimester screening tests you and your obstetrician would otherwise consider.
The biopsy illustrated
||At Day 3 of the embryo's development, a small hole is made in the outer layer of the embryo (the "zona pellucida") using a delicate laser beam.
||The embryo continues development until Day 5 or 6 when it becomes a blastocyst, characterised by the separation of cells into trophoblasts (which go on to become the placenta) and the inner cell mass (which go on to become the fetus).
||Trophoblasts are drawn out through the hole using a hollow suction tube called a biopsy pipette. The required cells are separated from the others using the laser and collected separately.
||The remaining cells quickly realign and