Embryo Grading: Factors Involved with Selection for Transfer

Embryo grading is a vital step during the process of In Vitro Fertilisation (IVF). It involves segregating, assessing and grading the spare or supernumerary embryos produced during the IVF. The grading of the supernumerary embryos ensures that good quality embryos are isolated to be used in embryo transfers or considered for embryo freezing (cryopreservation). Embryo grading thus increases the chances of successful implantation in fresh embryo transfers or at a later stage using frozen thawed embryos. Following are the factors that are taken into consideration while performing embryo grading.

Cell Count and Size

The first basic parameter on basis of which embryos are graded is the number of cells in the embryo. Within the first 48 hours following embryo fertilization, the embryo should ideally have only 2 to 4 cells, which should then grow to about 7 to 10 cells in the subsequent 72 hours. Apart from the number of cells, the size of the cells also is taken into consideration while grading embryo. Embryos that have cells of the same size are preferable and in most cases, cells that are almost of the same size are also considered for implantation.

Degree of Fragmentation

Another important attribute of the embryo cell that plays an important role in grading is the degree of fragmentation. An embryo is considered as fragmented when certain portions of the embryo cells (blastomerers) are broken and are separated from the nucleated portion of the cell. Fragmentation is also known as blepping. Embryos that have no fragmentation are given the highest grade, although in some special cases, fragmented embryos used for implantation have resulted in successful pregnancies and healthy babies. Normally, embryos whose cells display more than 25 percent fragmentation are not considered for embryo transfer.

Cell Division Rate

The rate at which the cells in an embryo divide and the consistency of the division is also considered an important factor in embryo grading. Usually, cells that display multiple nuclei are not considered for embryo transplants since research studies indicate that multinucleated cells are chromosomally abnormal. In extremely rare cases .when only a single embryo is available for implantation, multinucleated celled embryos have been used and have resulted in normal babies.

Granularity of Cells

The thickness of the outer layer, appearance of grains or granularity of the cells, and vacuoles also determine influence the grading of the embryos. There are multiple methods for selecting the best embryos for transfers to optimize the results of implantation while reducing the risks by multiple pregnancies. Methods such as Microscopic Embryo Grading grade embryos according to their visible and structural attributes. This method, however, does not assert the embryo competency or detect chromosomal abnormalities.

Genetic Factors

Specific methods such as Embryo Marker Expression Test (EMET) and Pre-implantation Genetic Diagnosis (PGD) are used to ascertain the health of the embryo and segregate the embryos that display specific genetic abnormalities or genetically inherited disorders respectively. This ensures only the normal embryos are used for implantation. Embryo competency testing (ECT) using Comparative Genomic Hybridization (CGH) is another method to reduce the risk of multiple pregnancies by transferring only embryos that display genetic integrity.