Preimplantation genetics is a screening procedure that can be performed during an IVF cycle to identify embryos with chromosome abnormalities or that contain markers for certain genetic disease and thereby avoid their selection for transfer. Preimplantation genetics can be performed in vitro at any of the following developmental stages, the zygote (day 1), cleavage stage (day 3) or blastocyst (day 5). At each of these stages, cell(s) are removed from the embryo for genetic testing in special laboratories. This does not appear to harm the ongoing development of the embryo with over 1000 healthy babies born worldwide after preimplantation genetic screening. However, an insufficient number of babies have been born to confirm that the procedure is completely without risk.
At the zygote stage (day 1), small cells called polar bodies that are naturally shed by the egg prior to and immediately after fertilization can be removed and they contain a complementary set of chromosomes to the egg. Polar body analysis is performed early in development but it can only tell us about the maternal genetic contribution of the subsequent embryo. Alternative methods that can reveal both parental genetic contributions involve either the removal of an embryonic cell (blastomere) from the cleavage stage embryo on day 3 of development or the removal of several cells from the outer layer (trophectoderm) of the blastocyst on day 5. Genetic testing is performed on these cells and typically a result is produced within 24-48 hours.
This testing is typically performed on a Day 3 embryo (eight cell stage) where one of the eight cells is removed and subjected to genetic scrutiny.
Indications for preimplantation genetics include couples at high risk for producing offspring with genetic disorders, such as carriers of cystic fibrosis or Tay-Sach’s disease. These couples may wish to prevent the birth of a child with an abnormal gene. In this case, special probes must be used in the test to detect the gene in question.
In IVF pregnancies of women 35 years or older, those with multiple failed IVF cycles and those with repeated miscarriages, research has shown that chromosomal abnormalities of the embryo increase either the risk of spontaneous miscarriage or the development of a genetically abnormal fetus. Performing preimplantation genetic screening for the 9 chromosomes most involved in chromosomal abnormalities (chromosomes 13, 15, 16, 17, 18, 21, 22, X & Y) could prevent the initiation of these abnormal pregnancies in IVF patients. An estimated 60 percent of all early miscarriages are associated with a chromosomal abnormality in the embryo. The purpose of preimplantation genetic screening is to select and transfer to the uterus only embryos that do not have numerical abnormalities for the 9 chromosomes tested in order to achieve higher implantation rates and fewer pregnancy losses.
Preimplantation genetic screening is only accurate approximately 90% of the time. This inherent 10% error rate means that there is no guarantee of a healthy baby. Even in the general population there is a 3-5% incidence of birth defects or malformations. In addition, preimplantation genetics can not detect abnormalities of chromosomes or genes which are not included in the test nor can it detect abnormalities if they occur in only some of the cells in the embryo (mosaicism). It is highly recommended that traditional prenatal diagnosis in the form of chorionic villous sampling or amniocentesis is performed if a pregnancy is established after preimplantation genetics.