What is Preimplantation Genetic Screening (PGS)?
Preimplantation genetic screening (PGS) tests embryos to determine if they are chromosomally normal. PGS can lower miscarriage rates since only chromosomally normal embryos are transferred. Women who have a history of recurrent pregnancy losses or of repeated implantation failures oftentimes benefit from PGS because it avoids the transfer of aneuploid embryos which result in miscarriages and can provide answers in cases of repeated IVF failures. Women who are of advanced maternal age or whose ovarian reserve is limited may choose to bank chromosomally screened blastocysts for the future or use PGS to minimize their risk of miscarriages. Finally, women who want to avoid twins yet maximize pregnancy rates with a single embryo transfer may also choose PGS.
Preimplantation Genetic Screening Process
In the past embryos were biopsied on the third day when they had 4-8 cells and only a handful of chromosomes could be tested. Now, embryos are biopsied on day 5 when they are blastocysts. Blastocysts are embryos which have hundreds of cells and have differentiated into the inner cell mass that develops into the fetus and the trophectoderm that develops into the placenta. On day 5 or 6, a few cells are taken from the trophectoderm and tested via array comparative genomic hybridization (aCGH) or next generation sequencing (NGS). Both aCGH and NGS assess multiple locations along the entire length of all 24 chromosomes to detect any chromosomal abnormalities. Implantation and pregnancy rates are higher when only chromosomally normal embryos are transferred but not 100%. Other factors that can impede success include structural integrity of embryos, and hormonal or uterine factors. Miscarriage rates are lower but miscarriages can still occur. Women are advised to undergo prenatal genetic screening if advised by their obstetricians because PGS screening does not preclude the need for prenatal genetic screening.