Genetic Diagnosis (PGD or PGS)
Pre-implantation Genetic Screening (PGS) is a technique that can be added to in vitro fertilization, oocyte donation and gestational surrogacy treatments. With this method, the prospective parents will practically eliminate the risk of some major chromosomal abnormalities, such as Down’s Syndrome (trisomy 21), trisomy 18 (three of chromosome 18, instead of two) and sex chromosome abnormalities of X and Y.
This procedure can also identify genetically abnormal embryos. There are many incurable diseases or disorders which are genetically based such as chromosome translocations and inversions, Cystic Fibrosis, Fragile X, Myotonic Dystrophy, Thalassaemia, Tay Sachs disease and others. Using PGD, it is possible to select the unaffected embryos for transfer.
PGS/PGD requires the creation of embryos by in vitro fertilization, oocyte donation or gestational surrogacy.
Process of Genetic Diagnosis
1. Removal of embryonic cells
Once embryos reach the 5 to 8 cell stage (three days after egg retrieval), an embryo biopsy is performed by creating an opening in the egg shell around the embryo. This is one technique for PGD biopsy. It allows a transfer in the same cycle (fresh transfer). Depending on the test being performed and the embryo quality, this may be one option for patients.
The other method for PGD analysis is to wait until the blastocycst stage which is typically day 5 or 6. The biopsy is done on a group of cells called the trophectoderm and the analysis is performed by another method. This type of analysis is typically more accurate. The results are ready in 1 to 2 days. The embryos need to be frozen the same day as the biospy and transferred at a later date in a frozen embryo transfer cycle.
2. Embryonic Chromosomal Analysis
After the biopsy on day 3, the embryos are placed back in an incubator to await the results of the genetic analysis. The biopsied cell’s chromosomes are analyzed outside of NOVA IVF in a PGD laboratory. The embryos themselves do not leave the NOVA IVF laboratory. The day 5 or 6 embryos are frozen by the vitrification method and a frozen embryo transfer is set up for a later date.
3. Extended Embryo Culture
The genetic analysis takes approximately one to two days. The embryos that were biopsied on day 3 will be typically transferred at the morula (day 4) or blastocyst (day 5) stage. Once the results of the PGD analysis are obtained, the embryos that did not show abnormalities can be transferred. If X and Y testing has been done, the gender could be selected as well.
The accuracy of PGD cell analysis approaches one hundred percent but it is not guaranteed. It is possible, even though highly unlikely, that embryo(s) that have tested as normal may not be genetically perfect.
So far, there is no evidence that PGD embryos result in an increased chance of abnormalities in the baby or that the risk of birth defects is higher than the usual risk of abnormalities (2-5%) when compared to conceptions that occur spontaneously.