PGD requires the biopsy or removal of cells from each embryo for analysis.
At Hollywood Fertility Centre, our advanced embryo culture techniques allow us to wait until the embryos have reached the optimum fifth day of development, when they can have 100 or more cells, and then remove 3 to 4 at a time. Other clinics conduct the biopsy at Day 3 of the embryos’ development when they consist of just 6 to 8 cells, and only a single cell is removed, reducing the number of opportunities for success.
By waiting until embryos have reached the blastocyst stage, Hollywood Fertility Centre 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.
Hollywood Fertility Centre scientists can use a number of different methods to analyse the biopsied cells. Traditionally, PGS was performed using a technique called fluorescent in situ hybridisation (FISH) which accounted for only a few chromosomes, compared to the 23 chromosomes that are present in the human embryo. FISH has several technical issues which made it a useful yet limited tool for screening.
During the last few years a more powerful technique using molecular tools has been applied for embryo genetic screening: array comparative genome hybridisation or aCGH. CGH allows accounting for all 23 pairs of chromosomes and the sex chromosomes, with a higher accuracy and less technical limitations.
If the problem is at a gene level rather than the chromosome, it is more common to use a technique called polymerase chain reaction (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.
The results for these tests are produced in 2 days (PGD) or 2-3 weeks (CGH). For this reason, every embryo tested will be vitrified and will remain in storage at Hollywood Fertility Clinic until the results are available to see if a healthy embryo is ready to be transferred in our patient’s next cycle.
Even in the blastocyst, the embryo’s cells are not in the final form they will have as a fetus and placenta after implantation. CGH for chromosome counting (pre-implantation genetic ‘testing’ rather than true ‘diagnosis’) is a procedure that reduces the chance of Down syndrome or having a miscarriage, for example. However, it does not eliminate these possibilities, so even if you have CGH, you should still have the usual first trimester screening tests you and your obstetrician would normally consider.back to top