Preimplantation Genetic Screening (PGS),
sometimes referred to as Preimplantation Genetic
Diagnosis (PGD), is a technique used in conjunction
with IVF in order to increase the likelihood of
transferring chromosomally normal embryos into your
uterus. Preimplantation Genetic Diagnosis is
indicated when a couple carries a gene for a
specific genetic disorder. Some examples include
cystic fibrosis, Huntington’s disease, sickle cell
disease, muscular dystrophy, polycystic kidney, Tay-Sachs,
and hemophilia. Patients may know that they are at
risk for certain genetic disorders, either because
one of the couples has the disease or because one
member of the couple is known to be a carrier for a
disease. This situation is called a “single gene
defect” These couples can have their chromosomes
evaluated in order to determine which alleles
(certain components of a gene) that they carry that
can lead to the development of an embryo with the
specific disease. The couple submits either a blood
or saliva sample to an outside reference laboratory
that collaborates with Austin IVF in order to
determine the presence or absence of the abnormal
gene. Once this has been confirmed, the reference
lab utilizes the genetic material from the couple’s
blood to create a genetic probe. This probe allows
the identification of the same abnormality from the
DNA within a single cell that is removed from each
embryo in the IVF lab.
On day 3 following the egg retrieval, each embryo
is biopsied and a single cell (blastomere) is sent
to the reference lab for evaluation. The reference
lab sends a report of the evaluation of the cell
from each embryo by day 5 so that unaffected embryos
can be selected for transfer. This allows for
selection and transfer of embryos that are
unaffected in order to avoid passing on the disease
to your children.
PGD is also helpful for an individual who is
genetically normal but may have a balanced
translocation of their chromosomes. In this
situation, parts of two unrelated chromosomes may
trade places with each other, resulting ultimately
in embryos that contain either too much or too
little DNA. As with single gene defects, PGD allows
for the identification of these “unbalanced”
translocations prior to the actual embryo transfer,
avoiding the likelihood of transferring embryos that
will either not implant or subsequently miscarry.
PGS has also been used to evaluate embryos from
couples who have experienced recurrent pregnancy
loss. The most common reason for the losses is
genetic and usually is associated with the embryo
having either too many or too few chromosomes, a
condition referred to as aneuploidy. Some conditions
that result from aneuploidy include Down’s syndrome
and Turner’s syndrome. These two abnormalities
typically produce abnormal pregnancies that usually
miscarry spontaneously. These are the two most
common aneuploidy abnormalities that occur.
Patients who may benefit from PGS include
patients with recurrent miscarriages, patients with
repeated implantation failures with IVF or couples
in which the woman is over 40 years of age. In PGS,
as with PGD, a single cell is biopsied from each
embryo on day 3 and the cell is sent to the
reference laboratory. The reference laboratory
analyzes the chromosomes within each cell to
determine which embryos have the proper number of
chromosomes. A report is sent to Austin IVF on day 5
and only those embryos with the proper chromosome
compliment are selected for transfer to the patient,
thus minimizing (but not eliminating) the risk of
aneuploidy.
