- Preimplantation Genetic Diagnosis -
How is PGD accomplished?
It is possible only in conjunction with IVF. The preparation and the
procedure is done in a standard fashion, involving the administration of
drugs to insure that several follicles will be available for retrieval
of several oocytes (eggs). The woman is given daily injections of
medication in order to achieve that.
At the proper time, the oocytes are recovered using vaginal ultrasound
guidance or laparoscopy. In an IVF laboratory the sperm is prepared
(capacitated) and added to the oocytes. After fertilization, the oocytes
now called embryos will start dividing. When proper stage of division is
reached, one cell (blastomere) is removed from each embryo and made
available for genetic evaluation. The removal of the blastomere is a
technically challenging procedure. The goal of the embryologist is to
remove an intact cell with minimal trauma to the remaining embryo. It is
accomplished using special microscope and micromanipulators. The
biopsied blastomere is delivered to the genetic laboratory and the
embryo returned into the incubator and appropriate tissue culture media
to continue development.
The genetic laboratory is faced with the difficult task to evaluate a
single cell for genetic disorder. For comparison, a laboratory examining
various tissues of an adult person has thousands, if not millions of
cells available from patients blood, biopsied tissue, or amniotic fluid.
Thus, with the current available technology, the PGD laboratory cannot
truly screen for multiple possible abnormalities. It can only identify
the presence or absence of a specific disorder.
Various methodologies are used for this purpose, depending on the
genetic problem known to exist in one or both parents. For this reason,
it is possible that the embryo might be diagnosed as not having a
particular abnormality questioned, but it might harbor another one,
which was not suspected and for that reason not tested for it. Even in
the best laboratory there might be some blastomeres which would be
impossible to diagnose one way or the other, that is, confirm presence
or absence of suspected abnormality.
Only embryos proven not to carry the genetic abnormality in question are
used for transfer or freezing. Those, which are known to be affected,
and those which cannot be determined are not transferred into the
Risks Of The Procedure
There may be a number of risks and/or possibilities of complications.
From the patient's point of view, even after going through their
treatment cycle, Preimplantation Genetic Diagnosis, and In Vitro
Fertilization, there is no certainty that the pregnancy will occur. In
most patients the IVF technology can produce embryos in vitro but after
they are transferred back into the uterus, no one can guarantee that
each embryo will implant.
Statistics indicate that younger patients have better chances for
successful implantation and ongoing pregnancy than older patients. In
general, the chances decline in mid-thirties and on. There are also
individual exceptions, when patients below the age of thirty-five might
be "poor responders" and produce only limited number of oocytes. In the
other hand, patients who are in high thirties or even forties might
respond very well and produce a significant number of oocytes ready for
fertilization. The general experience and statistics are only for
information. Patients should be evaluated individually.
When the In Vitro Fertilization procedure is combined with
Preimplantation Genetic Diagnosis there are some hadicaps and some
advantages. Obviously biopsy of the developing embryo does not make it
any better and some embryos migh not survive this procedure. However,
embryos which are found to be genetically normal have overall better
chance to implant and develop into ongoing pregnancy. It is also
believed that pregnancy rates with Preimplantation Genetic Diagnosis
might be better than in patients having the simple In Vitro
Fertilization. The reason is that women undergoing PGD frequently are of
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