Preimplantation
Genetic Screening (PGS) for Aneuploidy - 24 Chromosome Testing
Now Available at RCC
Preimplantation genetic screening (PGS) provides a means to screen embryos for abnormalities
in chromosome number prior to uterine transfer in an IVF cycle. PGS is
also an extremely accurate method for identifying male and female embryos making gender selection, or family balancing, a scientific reality. Many couples want to experience the joy of raising children of both genders.
The risk of genetic disorders is directly related to female age. Women over 35 years of age are at higher risk for chromosomal abnormalities, which can lead to early miscarriage or conditions such as Down’s syndrome.
In the PGS procedure, one or two cells are removed from the embryo in a procedure known as a blastomere biopsy
and genetically evaluated. The biopsy involves removing a small
sample (single cell) of the embryos DNA. Initially, the cells have not differentiated and a biopsy can be taken, usually on day 3, without damaging
the other cells and ultimately the embryo. The removed cell can
then be used for PGS testing. Results of the screening are typically available within 48 hours. This means that the embryos transferred have the opportunity to develop to the
day 5 or blastocyst stage.
Embryos have usually been examined using fluorescent in situ hybridization (FISH). FISH involves labeling chromosomes with fluorescent tagged probes and examining them under a special microscope. These tags
which are capable of evaluating (up to 12 pairs of chromosomes) allow
for gender determination and identification of missing or extra chromosomes
(in the 12 pairs evaluated).
Recently screening (2009) has become available through
Gene Security Network
for all 24 chromosome pairs in a single blastomere using
state-of-the-art microarray chips. In addition they offer
proprietary "parental support" testing to increase the accuracy
of the results. Prior to testing the blastomere, cells are
collected from the parents (or the egg or sperm donor) and a
sophisticated bio information algorithm is used to incorporate
parental data into the results on each cell. Parental support
helps correct errors produced during laboratory analysis, helps
detect DNA contamination, assists in determining parental origin
of trisomy or monosomy, provides detection of haploidy or
polyploidy and assists in ranking the results for transfer to
account for mosaicism. Test accuracy is thought to exceed 99%.
PGS does not guarantee a successful IVF cycle due to the fact
that their are many factors that affect the chance for success.
In addition, no test can guarantee the birth of a healthy baby.
There are limitations to all PGS tests because any single cell
analyzed may differ from genetically from the other cells in the
embryo. This condition is called mosaicism. It is currently
hypothesized that in some cases of mosaicism that the embryo can
"self correct" and thus after PGS some embryos may not be chosen
for transfer that may have developed into normal fetuses if they
had been used. This situation although feasible is probably
uncommon.
In our opinion, aneuploidy screening should be discussed and
considered in women over the age of 35, after recurrent
pregnancy loss, or after previously unexplained failed IVF
cycles. It can also be considered for decreasing the risk of
have a child with aneuploidy.
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