PGS is technically similar to PGD but instead of using the technique to find a specific familial disease the major goal of PGS is to detect which embryos have an abnormal number of chromosomes and are thus less likely to lead to the birth of a healthy baby. Therefore, the technique is used to screen the embryos so that the ones with a better chance of successful implantation and pregnancy are selected for transfer. While the theory of PGS has great appeal, the actual practice has not met these expectations.
Biopsy of an 8-cell embryo
The procedure of removing 1-2 cells from an embryo during biopsy in and of itself reduces that embryo’s implantation potential. Furthermore, a single cell may not be representative of the entire embryo inasmuch some early embryos may contain some normal and some abnormal cells (mosaicism). Embryos may have the capacity to set aside the abnormal cells and favor the development of normal cells or they may even correct the chromosomal abnormalities within the cells (self-correction).
Finally, the clinical value of PGS depends on the number of embryos available to test: its power to select better embryos increases when a large number of embryos are available for testing. Abnormalities in chromosome number (aneuploidy) increase with advanced maternal age (What is Age Factor?) so older women are the primary candidates for PGS. However, advanced age correlates inversely with the number of eggs and embryos produced (How Do We Test Ovarian Reserve?), i.e. older women have fewer embryos suitable for PGS. Therefore, the clinical value of PGS is lowest in the age group of patients who would most benefit from this technique.
In summary, PGS has been shown to decrease the number of miscarriages after IVF but to date it has not improved the live birth rate after IVF. At this point, couples with recurrent pregnancy losses associated with aneuploidy are most likely to benefit from PGS (Recurrent Pregnancy Loss). However, preimplantation genetics is a dynamic area of continuing research and future techniques may well overcome the shortcomings of the currently available methods such as FISH (fluorescent in-situ hybridization).