- October 21, 2017 at 16:43 #4021Parents LifeKeymaster
In the past, when a couple was vulnerable of having a child with a genetic condition, the only options for genetic diagnosis were a chorionic villous sampling or a genetic amniocentesis. These options are not optimal since terminating a pregnancy can be quite stressful thus, for many couples, are not considered as an options. Preimplantation genetic diagnosis (PGD) provides couples with an alternative choice. The refinement of micromanipulation techniques provides the ability to perform genetic diagnosis of a few cells that are taken off from the embryo before transfer. The first effective PGD was done in 1990 for a couple who have been vulnerable of having a child with cystic fibrosis. Since then, the expertise to do PGD is growing. It may be performed for autosomal recessive and dominant conditions. PGD is an emerging technology, and as more genetic probes become available, as increased demand in this procedure will be.
Preimplantation genetic screening (PGS) has become a well known adjunct to IVF treatment. As opposed to PGD, which detects specific mutations that induce a particular disease in a child, PGS can be used to identify whether an embryo has the normal 23 pairs of chromosomes. Many embryos come with an extra or missing chromosome (known as “aneuploidy”), which prevents them from implanting or leads to a miscarriage. Embryos using the normal chromosome complement (known as “euploidy”) possess a greater possibility of implanting (additional 15%-20%) when compared with untested embryos. PGS is usually done on blastocyst stage embryos. A few cells are biopsied before cryopreservation. Cells are delivered to the laboratory and the DNA amplified and the chromosome status is decided. This takes several days, so no fresh transfer is further performed. Only euploid embryos are transferred subsequently.
Cost was a major limiting factor to PGS usage in the past, but because of modern technologies (NexGen sequencing), the price continues to be reduced. A significant issue with PGS is “mosaicism” that refers to the fact that some embryos possess some abnormal cells that are eventually marginalized in nature along with a normal baby results. If these abnormal cells were biopsied, then the embryo could have been erroneously discarded. The incidence of mosaicism is not known but tend to range from 3% to 15%.
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