Embryo grading is a mechanism for testing the viability of embryos and deciphering the optimal day due for the transfer or freeze of the embryos. There are several embryo grading systems adopted in IVF labs. The result of the embryo grading provides the embryologist with information on what the embryos look like and the development condition of the embryos in the culture, the results inform Embryologists' choices of which embryos to transfer or which to cryopreserve.
Embryo grading results are not objective and reliable, They don't explain the internal genetic condition of the embryo. Research has revealed many cases of embryos with poor grades that make a pregnancy and cases where embryos with good grades don't make a pregnancy.
Grading of embryos come at different stages of the embryo's growth. Most of the transfers occurs on either of Day 3 or Day 5, the grading therefore take place on these days. The grading system for each day is totally different.
Day 3 embryos are the “cleavage stage” embryos. This is a stage where the cells in the embryo divide up but the embryo doesn't increase in size. As embryos divide, sometimes a small portion of the cytoplasm breaks off and forms a fragment. Embryos that contains a lot of fragmentation can be said to have developmental distortions. This is because in the process, the cells lose much cytoplasm in the creation of those fragments.
Day 3 embryos are graded on the basis of their cell lots and their look. Typically, a good, normally growing Day 3 embryo will contain about 6 to 9 cells. Embryos that have this number of cells have greater chances of developing into viable blastocysts than embryos with fewer cells.
A viable grade one embryo for instance, is one in which all of the cells are the same size and there is no fragmentation in the embryo. A system used to grade embryo appearance is:
Grade G for good
Cells are symmetric and no fragmentation is seen.
Grade F for fair
Cells are of symmetric; minor fragmentation only.
Grade P for poor
Cells are asymmetric with presence of no fragmentation or moderate fragmentation
Embryos between Grade G and F, according to this system, have better chances of making it to development to the blastocyst stage. Therefore, a 7-cell Grade F embryo might have same chances of survival than a 8 cell grade A embryo on Day 3. So basically, a quality grade of embryo at Day 3 will be a 6-9-cell grade A embryo.
The Day 5 embryos are the blastocyst stage embryos. The blastocyst stage of embryo development is the stage of where all embryos must attain to be able to get implanted in the uterus. The structure of the blastocyst is way complex when compared to the cleavage stage embryo.
Embryos at the blastocyst stage have their cells rapidly dividing, a blastocoel starts to form which causes the embryo to start to expand and increase in overall size. At the Blastocyst stage the embryos are worked into two parts: the inner cell mass, ICM (making up the fetus), and the Trophectoderm Epithelium (TE) cells, (developing into tissues needed to sustain the pregnancy, i.e. placenta).
The grading of embryos at the blastocyst stage, involves the allotment of a letter grade to each of the cell types to label how much expansion the embryo has undergone.
Early Blastocyst: The blastocoel fills almost half of the total volume of the embryo, but there is no expansion in the size of the embryo.
Expanded Blastocyst: The blastocoel fills more than half of the volume of the embryo and overall expanded with a thin zona in the overall size and notable thinning of the zona. The embryo contains 182 cells.
Hatching Blastocyst: Here, the trophectoderm begins to herniate through the zona. Few cells start hatching out of zona which is best stage for biopsy.
Fully Hatched Blastocyst: Contains more than 250 cells. Free blastocyst fully removed from the zona.
Inner Cell Mass (ICM)
Tightly packed compacted cells make up Grade A. Large, loose cells constitutes Grade B. Embryos with no ICM distinguishable is the Grade C.
A - Many healthy cells forming a cohesive epithelium.
B - Few but healthy cells, large in size.
C - Poor, unevenly distributed cells.
The detection of the viability of an embryo at Day 5 is achieved by combining the grading result of the components of the embryo.
Therefore, the best quality Day 5 embryo is one with a grade of AA, AB, BA or BB expanded blast.This is all you need to know about embryo grading and success rates. We hope we have helped you!