Couples with a family history of a genetic disorder and older
mothers are more likely to have a baby with genetic birth defects.
Preimplantation genetic testing and diagnosis (PGD) can help these
parents dramatically improve their odds of giving birth to a healthy
child.
 Embryos
that have certain genetic defects develop improperly. Used with
in vitro fertilization (IVF), PGD can help us select the best
embryos and avoid specific birth defects.
In PGD, a embryologist removes one or two cells from each embryo
created in the IVF cycle. The cells are tested for abnormal genes.
Only the embryos that have normal cells are transferred into the
woman.
Since PGD is not 100% reliable and only tests for specific defects,
parents should use other prenatal genetic tests, such as amniocentesis
or chorionic villus sampling. PGD is expensive and still considered
an experimental procedure.
Who Should Consider PGD?
PGD was originally developed to help couples with family history
of genetic disease such as cystic fibrosis, Tay-Sachs, hemophilia
and Down's syndrome. As PGD has become more refined we have found
PGD benefician to couples with recurrent pregnancy loss and repeat
IVF failure.
What are the Benefits?
Prior to PGD, many couples with a family history of severe genetic
disorder may have decided against having children. PGD dramatically
improves the odds of having a healthy baby without the disorder.
In some cases, biologists can see whether the embryo has the defect.
Some disorders only affect male offspring, so that female embryos
may be selected to avoid the condition even if the exact defect
isn't understood.
PGD allows analysis of the most common chromosomal abnormalities,
which are frequent causes of pregnancy loss and/or IVF failure.
For couples with recurrent pregnancy loss or repeat IVF failure,
PGD can help understand the cause of the problem and identify
the chromosomally normal embryos prior to embryo transfer.
What are the Risks?
One of the biggest risks to PGD is damage to the embryo. PGD
requires the removal of one or two cells from each embryo. The
embryo development is slowed slightly, but is otherwise normal.
Most embryos are not adversely affected by the procedure, however,
some embryos may be damaged during the removal.
Another risk to PGD is incorrect results. Incorrect embryo results
can occur in a small percentage of cases due to the very small
amount of DNA being analyzed. Incorrect results can come in two
forms:
False positive: An embryo that the PGD detects as abnormal
may be normal. This embryo would not be transferred, even though
it could have become a healthy baby.
False negative: An embryo that the PGD detects as normal may
be abnormal. This embryo would be transferred, and would result
in a miscarriage or child with birth defects. Because of this
risk, other genetic tests, amniocentesis or chorionic villus
sampling, should be performed.
How is PGD Performed?
The first step is to speak with your doctor to determine whether
PGD is appropriate for you. PGD has been found helpful for couples
with known genetic disorders, recurrent pregnancy loss, repeat
IVF failure and in some cases women greater than 35 years of age.
Just as in a normal IVF procedure, the next step is the administration
of medications to create eggs followed by the retrieval of the
eggs. The eggs are then fertilized with sperm and the embryos
are allowed to grow for three days. At this time the embryos are
typically six to eight cells.
The next step is the embryo biopsy. The embryologist forms a
small opening in the outer membrane of the embryo, the zona pellucida
(see picture above). This is a similar process to assisted hatching.
The technician gently removes one or two cells out of the embryo
through the hole. These cells are then tested for genetic abnormalities.
In most cases, all the cells of an embryo will have the identical
genetic makeup. Therefore, the tested cells will show the genetics
of the remaining, viable embryo. The remaining cells of the embryo
are young enough that they will form a complete, normal fetus.
Women over 35 are more likely to have eggs with an extra or missing
chromosome (aneuploidy). In these cases, the laboratory will examine
the cells to count the chromosomes that usually lead to severe
birth defects.
Each human chromosome has a number, except the X and Y chromosomes
that determine gender. The biologist uses a technique called fluorescence
in-situ hybridization (FISH) to attach a particular color to each
13, 16, 18, 21, X, and Y chromosome. The biologist counts the
spots of each color for each cell. Normal cells will have two
of each color for the numbered chromosomes, as well as two X chromosomes
(female cells) or an X and a Y chromosome (male cells).
For couples with a family history of a disorder, the laboratory
will test for the specific defect. The laboratory must first test
cells from the parent who has the disorder or may be a carrier
to determine the exact defect. The embryo cells are then tested
in a process that uses FISH to see if they contain that exact
defect. The test doesn't reveal other genetic defects.
It takes less than 24 hours to perform the tests so that the
patient follows essentially the same schedule as a standard IVF
cycle with embryo transfer on the 5th day after egg retrieval.
After the tests are completed the best embryos without the defect
are transferred into the woman's uterus as in a standard IVF cycle.
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