Embryo marker tests have been developed over the years in IVF practices to identify the best embryos for transfer into the uterus, but no test has been found to be 100% reliable. Conventional approach in selecting the better quality embryos still relies on the appearance of the embryos under the microscope by paying attention to certain characteristics. Microscopic evaluation of the embryos is done on the 3rd or the 5th day prior to transfer and the “good quality” embryos are then selectively transferred. In some cases, perfectly looking embryos may be chromosomally abnormal resulting in a negative outcome or a miscarriage.
One of the embryo markers called human leukocyte antigen-G (HLA-G) is a non-classic type I human leukocyte antigen produced by the embryo. It plays an essential role in the development of the embryo and has been identified in the culture fluid surrounding the embryos. When embryos are cultured for IVF, they release certain proteins into the surrounding area and also use up the nutrients present in the culture media. This interaction has been of interest for investigators because by identifying such molecules or quantifying them, it may be possible to identify good quality embryos.
Based on medical research, embryos derived from culture media expressing high concentrations of HLA-G exhibited improved cell division and implantation rates. We are able to grow embryos individually and measure the amount of proteins released by each embryo to their surrounding environment or media. The higher the concentration of the marker, better the quality of the embryo in most cases, but exceptions do apply. In some cases, the levels are so similar to each other that differentiation is impossible to make between developing embryos. In such cases, morphological assessment along with marker levels are used to decide on which embryos to transfer. Embryo marker tests need improvement due to their low predictability of better quality embryos and the lack of reproducibility in different laboratories.
We are now able to identify the best embryos using advanced genetic tests called PGD or CGH, which do provide information about the chromosomal make up of each embryo. CGH is a promising technology because it provides information about all 46 chromosomes. Embryo marker tests are non-invasive, but limited in clinical practice currently. Until their reliability is documented and pregnancy rates are improved, morphological assessment along with the option of doing CGH testing currently appear to be the most reliable methods.