PGD- Applications

Sometimes one or both partners may carry a genetic disease such as hemophilia.  Hemophilia is known as a “sex linked” disease and can only be transferred to female children. PGD allows the embryologist to screen embryos for several genetic diseases, and sex.

For example, a female embryo from a woman with hemophilia would not be transferred, thus eliminating the possibility of transmitting the disease to the child. Other genetic diseases that can be screened include Down's Syndrome, Tay Sachs Disease, Sickle Cell Anemia, and many others.

PGD- The Process

PGD involves taking a small biopsy from a day 3 embryo, produced from an IVF cycle, and examining it using fluorescent in situ hybridization (FISH) or the polymerase chain reaction (PCR).

FISH involves staining chromosomes and allows the embryologist to determine if an abnormal number of chromosomes are present, which is called aneuploidy (such as Trisomy 21 or Down’s Syndrome) and the sex of the embryo. Aneuploidy is a common cause of miscarriage. FISH is only able to count the number of chromosomes (including the X and Y to determine sex) and FISH is not effective for screening single gene defects such as hemophilia or sickle cell disease. 

However, women over the age of 35 are more likely to develop embryos with an abnormal number of chromosomes, thus increasing their risk of a miscarriage.  FISH can be helpful in this group of IVF patients by transferring only embryos with the normal number of chromosomes at the day 5, or blastocyst, stage.

PGD also employs the Polymerase Chain Reaction (or PCR) which is a technique involving a chemical reaction and is used for screening of single gene defects such as sickle cell or Tay Sachs disease. This reaction makes multiple copies of the DNA for examination. All the genes are located in the DNA and absent or defective ones can be found with PCR.  DNA probes are developed using the parents DNA to screen for specific genetic diseases. Embryos identified with the genetic defect are not transferred to the mother, thus eliminating the possibility of transmitting the disease. Conversely, the normal embryos are transferred on day 5 at the blastocyst stage. 

If there is a specific disease present in your family you should mention this to your fertility specialist. You might be a candidate for our North Carolina PGD program. As research continues, many genetic diseases are being added to the list that can be screened using PGD.

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