What is PGD?
Preimplantation Genetic Diagnosis (PGD) is a technology used to screen embryos created from IVF with ICSI for genetic diseases, before they are transferred back to the female’s uterus. PGD is used to screen embryos for single gene defects such as Tay-Sachs, Huntington Disease, Sickle Cell Anemia and Cystic Fibrosis. It can also test embryos for chromosomal disorders such as Down’s Syndrome and for X-linked diseases like Hemophilia. Additionally, PGD may be used for women who suffer recurrent pregnancy loss from chromosomal abnormalities and advanced maternal age. Screening and transferring only unaffected embryos reduces the rate of miscarriage and may help to alleviate the decision to terminate a pregnancy due to a genetic defect.
What happens during the PGD process?
In this process embryos are created by using ICSI for insemination. The embryos that fertilize and begin to divide are cultured to Day 3 where they are 8-10 blastomeres (cells) in size. At this point a hole is made in the zona pellucida of the embryo, very similar to embryo hatching, but it is used for a different purpose. Once the hole has been made in the zona pellucida a biopsy micro tool is used to gently remove one or two blastomeres from the embryo. This does not harm the embryo because at this point the cells have not begun to differentiate. Differentiation means that certain cells will develop into certain tissues of the fetus. The 8-10 blastomeres are composed of identical genetic material, and will continue to divide and grow properly even if one or two cells are biopsied. The incident of embryo damage during this process is very low, but does exist as in any other micromanipulation. Once the blastomeres are biopsied, they are processed according to the testing to be performed, and sent to a laboratory specializing in PGD analyses for definitive results.
How are the results determined?
Results from the PGD are obtained, and the laboratory is able to determine which embryos are not affected by the genetic disease being screened for. At that point, generally on Day 5, the healthy embryos are transferred back to the patient in anticipation of creating a viable pregnancy. An HCG beta is drawn 2 weeks from the date of egg retrieval to determine if a pregnancy has been initiated.
What else do I need to know?
As with any diagnostic procedure, PGD is not 100% accurate. Some embryos have mosaicism which means that not all the blastomeres are comprised of identical genetic material. In this event embryos which have been determined healthy may in fact be affected by the genetic disease. Patients may undergo an amniocentesis or chorionic villus sampling (CVS) to confirm that the fetus is negative for the genetic disease.
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