Embryo grading during the process of In-Vitro Fertilization (IVF) is done to ensure that the best quality embryos are selected for transfer to the uterus. Obviously, Grade 1 embryos are the best quality, but Grade 2 embryos are also good.
Considering that the aim now is to try and achieve singleton pregnancies where possible, to avoid the complications of multiple pregnancies, it then makes sense to select good quality embryos. These are the ones that are most likely to implant and go on to successful pregnancies and births.
Assessment of the Embryos
The embryos are assessed each morning from Day 2--the first morning after fertilization--at which time it should have 4 cells. It is then assessed again on Day 3, when it should have 6 to 8 cells. Transfer will usually take place on Day 2 or Day 3. Sometimes the embryo is allowed to progress to Day 5, when it forms a blastocyst, when the first cell differentiation occurs.
The specific factors taken into account when grading embryos are:
- The expected number of cells for the specific day.
- The size, shape, contour, smoothness and symmetry of the cells. In Grade 2, the cells may be uneven in size.
- Fragmentation of the cytoplasm. The degree of fragmentation required for the different grades vary according to the center at which the IVF is being performed. Some will classify Grade 2 embryos as having 5-10% fragmentation, and some as having up to 20% or 25% fragmentation.
- Nuclear content normal.
The number of cells in the embryo on a particular day is related to the rate at which cleaving takes place. Embryos which cleave at an earlier stage and at a higher rate appear to be associated with better success in terms of implantation, going onto pregnancy. The reason for this is related to the maturation of the cytoplasm and nuclei.
This is part of the reason why loss of cytoplasm by means of fragmentation plays such an important role. However, fragmentation also contributes to a difference in cell size, which makes a big difference to the grading and outcome. This appears to be related to the fact that the cells are not able to have adequate cell-to-cell contact.
It has been found in cases where more than a single embryo is being transferred, it is more effective to transfer embryos of the same grade where possible. So, two Grade 2 embryos will do better than a mixed grouping of four different embryos.