Until recently, preimplantation genetic diagnosis (PGD) was conducted on day three of the IVF cycle when embryos reach eight cells.A single cell could then be removed for analysis and tested for the six to 10 most commonly abnormal chromosomes (aneuploidy) that lead to inheritable genetic diseases. PGD was thought to increase pregnancy rates due to an increased likelihood of transferring a higher percentage of genetically ‘normal’ embryos back to the uterus.
In fact, implantation rates actually fell when preimplantation genetic diagnosis was utilized in an IVF cycle. This drop in implantation rate and the corresponding pregnancy rate is likely due to the trauma associated with removing one of the eight cells from the early day three embryos. In the past, PGD biopsy required that the embryo biopsy always occur on day three of the IVF cycle. This timeframe allowed the lab to isolate and analyze the genetic material and return the test results to the clinic by the fifth day of the IVF cycle. Day five is the optimal day to transfer embryos, now 60 to 90 cells in size, back into the uterus to maximize the chance for implantation and normal pregnancy progression. Newer technology now allows the embryo biopsy to occur on Day 5, at the blastocyst stage (60-90 cells), and removes two to four cells for testing. Researchers believe this new
PGD technique is less traumatic to the embryo and generates a more accurate sample for testing since more than one cell’s genetic material is analyzed.
The older technology, called FISH or Fluorescent In-Situ Hybridization, required 48 hours for screening, and was only able to test for 6
to 10 of the most commonly known genetic mutations, such as Downs Syndrome and Tay Sachs. Now with the newer testing method, called Micro array, we can test all 46 chromosomes.
Patients using this newer Micro array technology do have an additional hurdle to overcome. Since the Micro array biopsy is conducted on day
five embryos (60 to 90 celled embryos) and the results of the biopsy take 48 hours, the embryos must be frozen and transferred back to the patient’s uterus in a subsequent in-vitro cycle.
The Fertility and Sterility Journal Paper on PGD by William Schoolcraft of Colorado Center for Reproductive Medicine will address pregnancy rates after an embryo has been biopsied, frozen and thawed in what will likely be the landmark article on this newer technique. The study found excellent pregnancy rates and very low miscarriage rates. It is quite possible that in the near future many or most embryos will be tested and fewer will need to be transferred since we will know they are genetically normal.
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