PGD is an advanced clinical laboratory technique, used in conjunction with in-vitro fertilization, which allows the physician and embryologist to screen for an abnormal number of chromosomes, and other genetic “defects,” which lead to genetic disease.

PGD and “preimplantation” refers to the fact that the embryos are biopsied prior to placement into the uterus. This is known as a PGD blastomere biopsy and does not damage the embryo since it has not yet differentiated into different tissue types.

It is estimated that up to 60% of early pregnancy losses are due to genetic abnormalities, particularly aneuploidy (abnormal number of chromosomes). The success of PGD in improving IVF outcomes has been impaired by the fact that the embryo biopsy is performed on day 3 of embryo culture and then forty eight hours later the genetic analysis is obtained and embryo transfer is preformed on day 5.

PGD must be carried out in conjunction with IVF or ICSI. It consists of two main steps: an embryo biopsy (see below) and analysis of the biopsied cell by an independent laboratory.

Three days after the eggs have been fertilized they have a single cell (in rare cases two) extracted using micromanipulation techniques and equipment (see Micromanipulation), the cell is fixed to a glass slide or deposited in a small tube and sent to a reference laboratory for analysis. The embryos are placed back in the incubator. Two days later the laboratory sends back the results and the embryos are transferred typically on day 5 or day 6 at the blastocyst stage.

After biopsy, poor quality embryos have a lower probability of continuing their development. So, not all embryos will be of transferable quality.

EMBRYO BIOPSY

Briefly, it is of relevance in any discussion of micromanipulation techniques to mention the potential to biopsy both eggs and embryos. This approach is known as preimplantation genetic diagnosis (PGD) and enables the screening of both the unfertilized egg by removal of the first polar body, or the fertilized multi-cellular embryo by removal of one or more cells either at the 6-12 cell stage or from the trophectoderm of the blastocyst. This material can be probed for either genetic mutations or gross chromosomal errors. This technology remains in its infancy and can be of profound importance clinically, but at this time only for cases with very clear medically-defined needs. The biopsy procedure requires very exacting skills of the IVF laboratory, and the egg or embryo is not entirely free of risk during the procedure. Hence, couples whose offspring have a high chance of inheriting a genetic disorder may have their embryos screened. Women who are at risk of generating eggs with a high risk of chromosomal anomalies can benefit from having their eggs or embryos screened for chromosomal normality. While embryos can have their sex determined through this procedure, the SEMERT team considers it inappropriate to do so except in cases of sex chromosome-linked disorders.