 Preimplantation
Genetic Diagnosis (PGD)
Currently, PGD is the only way to test an embryo prior to
implantation to determine whether chromosomal abnormalities or
hereditary conditions are present.
PGD testing is recommended for:
- Advanced Age
- History of Recurrent Miscarriages
- Repeated IVF Failures
- Severe Male Factor
- Family Balancing (aka Gender Selection)
- Chromosome Rearrangements or Translocation
- Single Gene Disorders
- Alzheimer’s Disease
- Beta Thalassemia
- Cystic Fibrosis
- Fragile X syndrome
- Myotonic Dystrophy
- Sickle Cell Anemia
- Tay-Sachs Disease
What is Preimplantation Genetic Diagnosis (PGD)?
PGD is one of the Assisted Reproductive Technology techniques that
includes a genetic test of the oocytes (eggs) and/or embryos obtained
by undergoing In Vitro Fertilization (IVF) and before the embryos are
transferred into the patient’s uterus. PGD helps diagnose many genetic
disorders like aneuploidies (extra/missing chromosomes),
translocations, or single gene disorders.
How is PGD Performed?
The PGD process involves three steps:
- Embryo Biopsy
In this step, one cell (blastomere) is removed on day three of
embryo development. At this point, the embryo normally will be
composed of 8 cells. If necessary, the polar bodies (which are
products of oocyte maturation) are removed before the embryo divides.
- Blastomere/Polar Body Fixation
In this step, the genetic material of the biopsied blastomere or
polar body is fixed in order to keep the genetic make-up intact for
further processing.
3. The Genetic Test
Performed by either of two means:
A. Flourescent In-Situ Hybridization (FISH) is
a procedure that analyzes the cell’s genomes mainly for numerical and
structural chromosomal abnormalities. FISH utilizes highly specific
DNA probes that bind to specific chromosomes. Once the probe binds to
its target chromosome, it flouresces or lights up, and is then
interpreted by laboratory personnel using a state-of-the-art imaging
system. In the case of aneuploidy testing, usually five to nine
chromosomes are tested, depending upon the area of concern. The most
common chromosomes tested are: 13, 16, 18, 21, 22, X and Y because
abnormalities involving these chromosomes are largely responsible for
failed implantation, miscarraige, or children born with genetic
abnormalities and disease.
B. Polymerase Chain Reaction (PCR) is a
technique used by molecular biologists to amplify genetic material of
interest. PCR multiplies the DNA sequences contained in the polar
bodies or blastomeres so the single gene disorders can be detected.
Does Removal of a Single Cell Hurt the Embryo?
It is proven that the removal of a single cell from at least six-blastomere
embryo does not interfere with the normal embryonic development. The
embryo compensates for the removed cell. In case of polar body biopsy,
the biopsied cell is independent of the embryo, therefore no harm is
done to it.
Can PGD Increase the Chances of Pregnancy Through IVF?
The PGD technique enables the elimination of genetically defected
embryos, thus choosing even fewer number of healthy embryos for
transfer with a higher chance of implantation, pregnancy, and healthy
live birth. The opportunity of reducing multiple pregnancies is
definitely more attainable with PGD as well.
Does PGD Replace Prenatal Testing?
To completely eliminate the risk of misdiagnosis by PGD, it is
recommended that all patients pursue testing following PGD. There are
two common methods of diagnostic prenatal testing – chorionic villus
sampling (CVS) and amniocentesis. Upon request, our genetic counselors
will help you coordinate this testing.
Pacific Reproductive Center performs the most advanced procedures
for the identification of genetic disorders by PGD. Having a PGD lab
in-house is greatly minimizing the risk of misdiagnosis, reducing the
cost for the patients, and making the timing of treatment process more
flexible and more manageable. |
