Pre-implantation Genetic Screening (PGS), also sometimes referred to as Comprehensive Chromosome Screening (CCS), is a new and important technology that has revolutionised the world of IVF. Everyone considering IVF should have PGS on their radar screen should know the following facts.
•PGS was developed to address what has been one of the greatest challenges in ART, the inability of embryologists to identify chromosomally normal (euploid) embryos for transfer into the woman’s uterus in an IVF cycle. PGS involves analyzing the cells of the embryo to ensure that it has 46 chromosomes rather than a different number. This identiﬁcation is vitally important to the success of IVF as chromosomal errors in embryos lead to their failure to implant, and cause most miscarriages and all pregnancies involving chromosomal abnormalities. Only chromosomally normal embryos become healthy babies. At least ﬁfty percent of the embryos of women older than 35 may have an abnormal number of chromosomes (are aneuploid).
• In an IVF cycle without any form of preimplantation genetic screening, embryologists make their best assessment of which embryos are chromosomally normal. They try to inform their assessment by using a microscope to visually assess embryo morphology (how they look). But “eyeballing” embryos under a microscope is an imperfect method of assessing which embryos are the best bet for transfer. Even the most experienced embryologists are unable to identify embryos that are chromosomally normal with their eyes because chromosomally abnormal embryos often appear to be perfectly normal.
•In an effort to develop a more precise method of screening for aneuploidy, scientists originally developed a technique called FISH (ﬂuorescent in situ hybridization). FISH was the ﬁrst technique to label chromosomes in an embryonic cell, but the problem with FISH is that only some chromosome pairs are examined. The remainder are left unexamined and this limitation is a signiﬁcant drawback.
• PGS was developed to address the limitations of FISH. It offers more “comprehensive” chromosome screening because all 23 pairs of chromosomes in each day 5 or 6 (blastocyst stage) embryo produced in an IVF cycle are examined following the biopsy of a few cells removed from the outer layer (trophectoderm) of each blastocyst.