Chioma Umeha
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.
