Preimplantation Genetic Diagnosis

Preimplantation Genetic Diagnosis

PGD is a procedure used to test embryos for either chromosomal abnormalities or inherited genetic disorders before uterine transfer and the establishment of pregnancy. Therefore, it helps alert couples to the possibility of a problem before the actual implantation of an embryo. PGD is usually used in conjunction with IVF and possibly ZIFT. This diagnosis enables a couple to discover if their child is abnormal before pregnancy, and to avoid having to contemplate the possibility of abortion later during the child-bearing term. PGD can detect approximately 203 inheritable disorders, some of these being cystic fibrosis, Tay-Sachs, beta-thallassemia, sickle-cell anemia, Gaucher's disease, Duchenne muscular dystrophy, spinal muscular dystrophy, myotnoic dystrophy, Lesch-Nyhan syndrome, Hemophilia A and B, fragile X-syndrome, and Kleinfelter's syndrome. Chromosomal abnormalities that can be detected include Down's Syndrome, Turner's Syndrome, known translocations and inversions, and problems in most of the 23 known chromosomes.

PGD is performed by removing a single cell from an embryo. The cell is usually removed from the embryo at the 8-cell stage or later on during development. The biopsied cell is then analyzed for chromosome abnormalities using a panel of chromosomes or for a single gene known to cause an inheritable disorder. Results become available after approximately 24 hours, and then the embryo can be transferred at the blasotcyst stage. The chance of pregnancy after PGD is performed on the embryo is still comparable to cases where the test is not undertaken, therefore it does not seem to negatively affect implantation. There is a 3-8% chance that the test results are erroneous. Therefore, both chorionic villus sampling and amniocentesis are recommended as confirmatory procedures that should be used to ensure normal pregnancy.

PGD should be considered by couples when there is a possibility of passing an inheritable genetic disease, when an individual is aware that they are carriers of a chromosomal abnormality that can be transmitted, when recurrent pregnancy loss has occurred, when IVF has failed repeatedly, and in couples where the woman is over 38 years of age.

PGD is also used for gender selection (choosing the sex of the fetus) by examining the chromosomes (XX for the female and XY for the male) of the embryos that will be transferred to the uterus. It is vitally important in cases where the genetic disorder is linked to gender (for example, Hemophilia, which affects the male offspring).