What are the chances that an abnormal embryo is going to be missed by PGD?
Remember that an embryo can have many different types of abnormalities. Preimplantation genetic diagnosis is only going to test for a specific type of abnormality. For instance, testing to determine if an embryo will produce a baby with Down’s syndrome (caused by three copies of chromosome 21) will not rule out the possibility that the embryo also has a gene mutation that would cause the baby to have cystic fibrosis. Furthermore, when we do chromosome testing, we can only look at a maximum of nine chromosomes. Since an embryo has 23 pairs of chromosomes, it means that we won’t be testing the remaining 14. We choose the nine that we do test very carefully, however. We believe that we can identify about 85-90% of the numeric chromosome abnormalities by testing the nine. In other words, we expect to miss a chromosome abnormality about 10-15% of the time.
