Preimplantation genetic diagnosis, or PGD, is a technique to detect defects in the early stages of an embryo, before it is transplanted to the mother's womb. PGD may be performed employing several methods, including the FISH analysis, the PCR analysis or the comparative genomic hybridization (CGH). Presently, CGH is considered the least dangerous for the embryo and gives more accurate results about the composition of the embryo.
Comparative Genomic Hybridisation
The comparative genomic hybridization is the newest development in the preimplantation genetic diagnosis. PGD is recommended in the IVF procedure, especially in cases of advanced maternal age or when one or both partners may carry genetic defects that can be transmitted to the baby.
The comparative genomic hybridization is applied on the embryo, and this is the only technique that can be applied when the embryo is in a more developed stage (5 days), when there are around 100 cells in the embryo. The analysis can be performed on several cells, so it is more likely to identify a chromosomal defect.
The CGH is performed using colored dyes which are applied on the embryo. The nucleus of the embryo will have a color and all the other cells will receive a different color code. The colors are then contrasted and if any of the colors are in excess, there is an extra or a missing chromosome, which means that the embryo is not healthy.
The entire comparative genomic hybridization analysis takes about 72 hours. The embryo will have to be frozen, so that it can preserve its quality and may be used to complete the IVF procedure. The speed of the CGH protocol may increased, but this is not recommended.
Comparative genomic hybridization is rarely performed, as it is only available in a limited number of clinics. However, in the future, as PGD is further developed, the technique should be made available in several more fertility clinics.
Advantages of CGH
The comparative genomic hybridization has several advantages over the FISH or ICSI techniques:
- Given that the CGH test is performed at a later stage of the development of the embryo (when it is 5 days old as opposed to 2 or 3 days), the embryo will have more cells (approximately 100) and the information on the embryo will be more accurate
- The embryo will be stronger and the analysis is less likely to cause any damage
Disadvantages of CGH
Given the fact that the comparative genomic hybridization can be performed on day 5 of the embryo, the introduction of the embryo will be delayed. The embryo shouldn't be kept for more than 5 days without being transferred to the mother; the embryo will have to be frozen, so that it will still have a chance of being implanted in the mother's uterus. However, through freezing, about 30 percent of the viable embryos may be lost, so this can be a problem when the number of viable embryos is low.