In some cases, genetic screening and/or testing of embryos may be indicated. Genetic testing of an embryo requires embryo biopsy, a procedure which is performed by an experienced, skilled embryologist in order to obtain cells from an embryo that can be analyzed for preimplantation genetic screening/preimplantation genetic diagnosis (PGS/PGD). The embryo’s cells can be sent for karyotype, also known as chromosome analysis, where chromosome number is counted and the arrangement of the chromosomes can be observed as well – this is known as PGS. If the embryo’s cells are to check for the presence or absence of a “single gene disorder” (cystic fibrosis is an example), then this process is called PGD.
In the recent past, embryo biopsies had been performed at the day 3 stage of embryo development. However, in recent years, day 5 embryo biopsy has been shown to be the preferred time for embryo biopsy because day 3 embryo biopsies have a relatively high rate of inaccurate results due to the potential for an embryo at that stage to have “mosaicism”, which means that there may be several different cell lines present within the day 3 embryo that have different karyotypes (for example, one cell line may be normal and the other may be abnormal in terms of karyotype) and the embryo may self-correct as it grows further or it may not self-correct. After day 3 embryo biopsy, it usually takes 48 hours to have the results of genetic testing, so the day 3 embryo must not only have normal results (which may be in question due to mosaicism) but it also must reach blastocyst stage. Even with survival to the blastocyst stage, there is a 40% decrease in implantation potential for embryos that have been biopsied on day 3. Therefore, if an embryo has to reach the blastocyst stage regardless of day of embryo biospy, it makes sense to biopsy on day 5 or 6 provided that the embryos are candidates to be grown to the blastocyst stage.
Currently, it is recommended that embryo biopsy be performed on day 5 or day 6 of embryo development, also known as the “blastocyst” stage of embryo development. This process involves removal of cell from the trophectoderm, which is the tissue that can become placenta if the embryo implants after embryo transfer to the woman’s uterus. Trophectoderm can be visualized at the blastocyst stage. Since the cells removed are not directly from the “inner cell mass”, which can become a fetus after the embryo implants, trophectoderm biopsy minimizes the procedural risk to the embryo. The genetic test results are significantly more accurate due to a minimized chance of mosaicism at the blastocyst stage of embryo development and the implantation potential of the embryos is not thought to be diminished by the procedure.
To watch a video of day 5 trophectoderm biopsy at Westlake IVF, please visit http://www.westlakeivf.com/