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Embryo Grading

What is embryo grading?

The quality of the embryo, or fertilized egg, is one of a number of important components in the success of an in vitro fertilization (IVF) pregnancy. There are several types of information used to evaluate the health of embryos, including the number of cells present and the grade, or quality of the embryo. Assigning a grade is one way to identify the best quality embryos for the selection of embryo transfer during an IVF procedure. Using this information, determinations are made about how many embryos to transfer, how many to freeze, and what to do with embryos that are not developing well.

Between 16 and 18 hours after the eggs and sperm are placed together it can be determined whether or not fertilization has occurred. The embryo is then allowed to develop for another 24 to 36 hours. Forty-eight hours after fertilization, the clinician evaluates the size and number of the divided cells, as well as the degree of fragmentation, which is a process in which portions of the embryo's cells have broken off. It is preferable to have little or no fragmentation; the more fragmentation seen, the poorer the embryo quality. Also evaluated is the thickness of the shell surrounding the embryo, and the number of nuclei per cell. These, along with other indicators, help to determine the viability of the embryo.

What is the system for embryo grading?

Selection criteria or grading systems must be applied for all stages of embryo developed from days two through four in order for the most viable embryos to be selected for transfer. Embryo development is evaluated approximately every 24 hours. Embryos are graded, or evaluated, by their appearance under the microscope for the number of cells, the characteristic of cells and the presence or absence of fragmentation.

Good quality embryos divide rapidly, have equal number of cells with clear cytoplasm, and have only few fragments. Some IVF clinics classify the embryos into grades one through four. Grade 1 includes the best quality embryos. These have a higher chance of implantation than those of grade 4. Research has shown that up to one-third of embryos are genetically abnormal. There is no guarantee that a normal looking embryo will be genetically normal.

The four level grading system for multi-cell embryos is evaluated in the following way:

  • Grade 1: even cell division, little to no visible fragmentation
  • Grade 2: even cell division, small fragmentation (less than 20%)
  • Grade 3: uneven cell division, moderate fragmentation (more than 30%)
  • Grade 4: uneven cell division, excessive fragmentation (more than 50%)

When selecting embryos for an embryo transfer, the most viable embryos are typically chosen. Those considered most viable are embryos that are rapidly dividing with little or no fragmentation. Therefore, embryos that have reached the most advanced developmental stage and have the least cellular fragmentation (Grades 1 and/or 2) are selected for transfer. Some labs may use a different system, so please check with your lab to learn which grading scale it uses.

What is the average life cycle of an embryo?

Day 0: Eggs are retrieved and combined with retrieved sperm
Day 1: Physician will check for fertilization
Day 2: Embryo should be fertilized and have 4 cells
Day 3: Embryo should now have 8 cells
Day 4: Embryo is now in morula stage and ready for embryo transfer
Day 5: Blastocyst stage of development

What is a blastocyst?

Multi-cell embryos that are graded level 1 or 2 often develop to the blastocyst stage. A blastocyst is an embryo that has developed to the point of having two different cells and a fluid cavity. They usually reach the blastocyst stage within five days after fertilization. Blastocysts are graded differently than younger embryos, with a number and two letters.

When grading a blastocyst, the number represents how much it has expanded (1 is the least expanded, 6 is the most expanded). The first letter (A, B, or C) lets patients know about the quality of the cell – this is essentially the part of the blastocyst that is going to be the baby. The second letter refers to the quality of the part of the blastocyst that is going to be the placenta.

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