The PGD process
What happens in PGD?
PGD requires the biopsy or removal of cells from each embryo for analysis. At Superior A.R.T., 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, Superior A.R.T. 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.
Superior A.R.T. scientists can use a number of different methods to analyse the biopsied cells. Comparative Genome hybridisation (CGH) is used to count the number of all 23 pairs of chromosomes and to observe how they are arranged. If the problem is at a gene level, rather than at the chromosome level, it is more common to use a polymerase chain reaction based method (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.
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 'testing' rather than true 'diagnosis') is a screening 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 cell 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 the embryo goes on developing.|
See a video of a blastocyst biopsy