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- Preimplantation Genetic Diagnosis - P.G.D. -

 

 

How is PGD accomplished?

 

 

It is possible only in conjunction with In Vitro Fertilization (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.

Genetic Evaluation

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 uterus.

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 handicaps and some advantages. Obviously biopsy of the developing embryo does not make it any better and some embryos might 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 "proven fertility".

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Last modified: 06/09/04