Pre-Implantation Genetic Diagnosis (PGD) and Screening (PGS)
Every parent’s wish is for a healthy child. This goal is complicated when there’s an increased risk of an inherited condition or chromosome abnormality. Preimplantation genetic diagnosis (PGD) and preimplantation genetic screening (PGS) provide the possibility of a solution. PGD and PGS help examine an embryo for the problem, even before a pregnancy begins.
PGD and PGS starts with the IVF process, where egg and sperm are brought together in the lab. Five or six days after fertilization, a biopsy is performed, where a small sample of cells is removed from each embryo. These cells are tested for the condition of interest and the embryos predicted to be unaffected are transferred to the woman’s uterus at a later point in time. PGD is the process of detecting a genetic syndrome, such as Huntington disease or muscular dystrophy. PGS identifies chromosome abnormalities like Down syndrome. More detailed information about different types of PGS can be found below.
The biopsy of the embryos for either PGD or PGS is performed at PCRM by our skilled embryology team. The cells are sent to one of two specialized laboratories for analysis. After the biopsy, embryos are vitrified and stored until the genetic testing is completed. PGD/PGS results are ready about 7 days after the biopsy.
In a standard IVF cycle, embryos are graded and selected for use by their appearance or morphology. When an IVF cycle includes PGD or PGS, the selection of embryos is based on their genetic makeup. Embryos predicted to be unaffected can be transferred back to the uterus at any time, even as soon as the next cycle.
Couples may choose to pursue PGD or PGS for a number of reasons:
- They have undergone standard IVF cycles that have been unsuccessful or resulted in miscarriages.
- They have experienced miscarriages due to an inherited chromosome rearrangement, such as a translocation or inversion.
- They have a child with a genetic condition and would like to reduce the chance of inheritance for their future children.
- An adult has a genetic condition or has an increased risk of developing a genetic condition and would like to reduce the chance of inheritance for his or her future children.
In current IVF practice, PGD and PGS are offered only in unique circumstances and in a select group of high-risk patients. The potential benefit of these techniques is determined during the first consultation visit with your fertility specialist.
If you would like more information about PGD or PGS, you can call PCRM to make an appointment to talk to our genetic counselors. For more detailed information regarding PGD or PGS, you can also refer to the web sites of Genesis Genetics (www.genesisgenetics.org) and Natera (www.natera.com), the laboratories to which PCRM sends biopsied cells for testing.
Types of PGS
Altogether, there are 46 chromosomes in the human cell. Not too long ago, the standard PGS analysis was able to examine only a few chromosomes. Advancing genetic technology now allows scientists to assess the entire set. Comprehensive Chromosomal Screening (CCS) refers to any type of PGS that assesses the complete chromosome complement, all 46 human chromosomes.
Evaluating an embryo’s chromosome make-up using CCS techniques helps to select the ones that have the correct number of chromosomes. Transferring these embryos may result in higher implantation and pregnancy rates and may help avoid miscarriage.
There are a number of different CCS platforms including array CGH, SNP array and quantitative PCR. Each technique ultimately performs the same task. All are highly effective, with accuracies of 97-99%.
For the majority of patients undergoing PGS at PRCM, the biopsied cells are sent to Natera. Natera’s CCS method is a SNP microarray platform coupled with parental bioinformatics. The results provide highly accurate information for all 46 chromosomes.