By Independent
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.
