genetic testing

In procedures such as in vitro fertilization (IVF) embryos are created in a laboratory and then transferred to the woman’s uterus. For some couples, especially those at risk for genetic disease, questions arise about the chromosomal make-up of those embryos. A technique called preimplantation genetic diagnosis (PGD) can provide information about some of those abnormalities.

• what is PGD?
• what can PGD detect?
• the PGD procedure
• success rates

what is pgd?

There are two types of PGD. The first technique screens couples who have a known risk for inherited diseases such as cystic fibrosis, Tay-Sachs, muscular dystrophy and chromosomal translocations among others. The second screens for aneuploidy to test for common chromosomal abnormalities associated with advanced maternal age, repeated IVF failures or recurrent pregnancy loss.

PGD is a highly sophisticated procedure that is offered by a limited number of facilities. Some centers limit its availability to couples who have experienced repeated miscarriages due to genetic disorders or who have already had one pregnancy/child with a genetic disorder and face the same high risk in subsequent pregnancies. Some centers make the procedure available to women of advanced reproductive age.

Since PGD is time sensitive, only available in a few clinics and the process requires advanced preparation, couples considering PGD should make arrangements well in advance of their IVF cycle.

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what can pgd detect?

PGD can identify the specific genetic abnormalities listed below or chromosomal abnormalities such as Down’s Syndrome. The gender of an embryo can be determined with PGD. Therefore, couples at risk of having children with gender-linked diseases, such as the male-specific hemophilia, X-linked mental retardation, or muscular dystrophy, can use PGD to identify, then transfer, only embryos that are not at-risk. As PGD procedures are refined, the number of disorders that can be diagnosed continues to increase, and the potential for diagnostic errors decreases. Couples should contact their IVF centers about whether PGD is available for a specific condition.

A sampling of the disorders PGD can detect includes:

Cystic fibrosis	Marfan's syndrome
Charcot-Marie-Tooth disease	Muscular dystrophy
Down syndrome	Sickle cell anemia
Duchenne muscular dystrophy	Spinal muscular atrophy type I
Franconi's anemia	Tay Sachs disease
Huntington disease	Thalassemia

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the pgd procedure

During PGD a cell is removed from an embryo when it has grown to eight cells, usually three days after fertilization. That cell is analyzed for certain genetic conditions using one of two basic techniques: fluorescent in situ hybridization (FISH) used to detect selected chromosome numbers, or polymerase chain reaction (PCR) based DNA amplification used to detect gene mutations. Results of those tests are generally available within 24 hours, at which point a decision can be made as to which embryos should be transferred. This should increase the chances of having a healthy baby.

Even though the risk of actually having a pregnancy with a chromosomal abnormality is low, not all embryos produced will have the entire chromosomal make-up needed to result in a healthy baby. Most of those embryos will not develop or implant. Studies have shown that if a woman is 30 years old, approximately a third of her embryos will not have the proper chromosome make-up. This is why the natural pregnancy rate even in fertile women may only be 20% to 30% a month. As a woman gets older, more embryos will have abnormal chromosome numbers so that by the age of 40, approximately 70% of a woman’s embryos may not have the right number of chromosomes to develop into a healthy child. Yet the risk of actually having a child with a chromosomal abnormality is still less than 2%. That means that many of these pregnancies will stop developing at early stages or will miscarry if they do implant.

PGD for aneuploidy screening can help identify those embryos that have tested normal for the most common chromosomal abnormalities.

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success rates

The American Society for Reproductive Medicine and the Society for Assisted Reproductive Technology states that, worldwide, at least 1,000 babies have been born following PGD. The report also states there have been no reports of increased malformation rates in fetuses analyzed through PGD. (When the procedure has been in use for a greater length of time, long-term studies may become available).

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