Later, a biopsy of the embryos that are available from IVF follows. A cell from a 6–8 cell stage embryo is retrieved (which corresponds to about three days after fertilization), and subsequently, embryos affected by the disease are excluded from the transfer.
Pre-implantation genetic screening (PGS)Pre-implantation genetic screening (PGS) is a broader, quite promising method for old couples and couples with repeated miscarriages or other unsuccessful attempts. We briefly explain the principles of this method.
During conception, 23 female and 23 male chromosomes meet each other and create a healthy 46-chromosome embryo. The division of chromosomes is called meiosis and gets more difficult to achieve as age advances. This explains why younger women are more fertile than older women and clarifies the reasons of higher risk for genetic disorders, like Down syndrome, in older pregnant women. Chromosome abnormalities are mainly naturally selected and do not survive conception or implantation, or they end with a miscarriage.
Defects, such as trisomies, monosomies or other combinations, may occur in any chromosome. These defects are called “aneuploidies”.
Screening provides information on the number of such chromosomes. For this reason, a cell is extracted from a 3-day embryo, and embryos with chromosome abnormalities are excluded from transfer. Despite some reservations, e.g., the fact that in some cases defects cannot be detected or defects are erroneously detected on healthy babies, the aneuploide screening is a successful method.
There are two current methods of aneuploide screening: