What is pre-implantation genetic screening (PGS)?
Preimplantation Genetic Screening (PGS), also known as Comprehensive Chromosome Screening (CCS) refers to optional testing that is performed on embryos prior to transfer to the uterus. Mount Sinai Hospital (MSH) was the first lab in Canada to offer PGS. PGS determines which embryos from an IVF cycle have the correct number of chromosomes so that those embryos can be selected for transfer. Such embryos have a higher chance of implantation and the resulting pregnancy has a higher chance of success.
PGS may help to:
- Reduce the chance of having a miscarriage
- Reduce the risk of an abnormal pregnancy
- Reduce the time and cost to achieve a healthy baby by helping choose the best embryo(s) to transfer
- Reduce the risks associated with the transfer of multiple embryos at a time
How does PGS work?
PGS is used in conjunction with in vitro fertilization (IVF) and uses the most sophisticated and scientifically validated technology. PGS examines all 24 chromosomes, the 22 non-sex chromosomes, plus the two sex chromosomes (X & Y) for any gains or losses, otherwise known as aneuploidy. Only embryos with the correct chromosome complement will be transferred. Sometimes PGS detects what is known as mosaicism which describes a situation in which different cells in the same embryo have different numbers or arrangements of chromosomes. When people only have mosaic embryos available for transfer, genetic counselling is advised prior to embryo transfer.
Regardless, all patients who are interested in PGS will meet with a Genetic Counsellor to ensure they are fully informed before moving forward with an IVF-PGS cycle.
What is the process for PGS?
Those interested in PGS should be referred to Mount Sinai Fertility. An appointment with a Genetic Counsellor will also take place.
In terms of the cycle itself, the entire procedure consists of five different steps, usually performed by different experts and laboratories:
The entire procedure consists of five different steps, usually performed by different experts and laboratories:
- Patients having PGS undergo ovarian stimulation and IVF-ICSI to create embryos. This part will take place at Mount Sinai Fertility.
- The second part involves embryo biopsy performed by an embryologist at Mount Sinai Fertility. The biopsy, which takes place at the appropriate stage for the ordered testing, is done to remove material from the embryo.
- The biopsy samples are then transported to the appropriate genetics reference laboratory, one of which is through Mount Sinai Services.
- The genetics reference laboratory analyses the biopsy samples.
- Should there be embryos for transfer, this will take place at Mount Sinai Fertility no sooner than the next month after the IVF cycle. Usually the embryos are frozen after biopsy. A frozen, thawed embryo transfer is planned in the next month if genetic testing demonstrates at least one unaffected and/or chromosomally normal (euploid) embryo for transfer.
Is PGS 100% accurate?
PGS, although highly accurate, cannot guarantee a chromosomally normal baby. Like any test, PGS is not 100% accurate and does not replace the prenatal screening that is offered to all pregnant individuals in Canada.
Does PGS guarantee a successful IVF cycle and a healthy baby?
No. There are many different factors affecting the chance of success after IVF. Transferring an embryo that has undergone PGS does not guarantee a pregnancy. It also does not guarantee a healthy child. Any pregnancy has a 3-5% risk for having a baby with birth defects. Most birth defects are not related to chromosomal imbalance and many of these are not detectable prior to implantation or even during pregnancy.