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 testing (PGT) for monogenic/single gene defects (PGT-M) and aneuploidies (PGT-A) provide the possibility of a solution. PGT-M and PGT-A is the modern terminology, but in the past this has been referred to as preimplantation genetic diagnosis (PGD) and preimplantation genetic screening (PGS) or comprehensive chromosome screening (CCS).
PGT-M and PGT-A help examine an embryo for a problem, even before a pregnancy begins. PGT 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. PGT-M is the process of detecting a genetic syndrome, such as cystic fibrosis, Huntington disease or muscular dystrophy. PGT-A identifies chromosome abnormalities like Down syndrome. More detailed information about different types of PGT can be found below.
The biopsy of the embryos for either PGT-M or PGT-A is performed at PCRM by our skilled embryology team. The cells are sent to a specialized laboratory, typically Cooper Genomics, for analysis. After the biopsy, embryos are vitrified (frozen) and stored until the genetic testing is completed. PGT results are ready about 1-2 weeks 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 PGT, 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 PGT 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.
The potential benefit of these techniques is determined during the first consultation visit with your fertility specialist.
If you would like more information about the benefits and limitations of PGT, you can call PCRM to make an appointment to talk to our genetic counselors. For more detailed information regarding PGT, you can also refer to the web sites of Cooper Genomics and Natera, the laboratories to which PCRM sends biopsied cells for testing.
Types of PGS
Altogether, there are 46 chromosomes (23 pairs) in the human cell. Not too long ago, the standard PGT-A analysis was able to examine only a few chromosomes. Advancing genetic technology now allows scientists to assess the entire set for missing and extra whole chromosomes as well as large missing and extra pieces of chromosomes (deletions and duplications).
Evaluating an embryo’s chromosome make-up using PGT-A 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 PGT-A platforms including array CGH (comparative genomic hybridization), SNP (single nucleotide polymorphism) array and NGS (next generation sequencing). Each technique ultimately performs the same task. All are highly effective, with accuracies of 97-99%.
For the majority of patients undergoing PGT at PCRM, the biopsied cells are sent to Cooper Genomics. Cooper Genomic’s PGT-A method is a NGS platform. The results provide highly accurate information for all 46 chromosomes.