Ever wonder how embryos make the grade in an IVF lab? Getting pregnant with advanced reproductive technology requires a little extra homework to understand the IVF process, and criteria embryologists use to “grade” embryos.
Just as a student can receive a high or low mark on their studies – showing how well they performed – embryos can be judged as to how well they will develop when transferred back into the uterus.
Grading Embryos on Day 3
On day 3 the embryologist, using a microscope, scrutinizes an embryos’ physical appearance to determine the ones that are most likely to progress to a pregnancy.
Embryologists look at the following characteristics in a Day 3 Embryo to decide its grade:
- Number of Cells: Healthy Day 3 embryos will have between 6 and 10 individual cells surrounded by a “shell” or the ZP, zona pellucida. Embryologists know from studies that embryos with 6 to 10 cells are more likely to produce a healthy pregnancy than embryos with fewer cells.
- Overall shape of the cells: Embryos with equally divided cells have a better chance to produce a pregnancy than embryos with unevenly divided cells. When a cell divides, the chromosomes line up in the middle and the cell divides in half to form two new cells. Some cells don’t split evenly down the middle. Uneven division can be an indication of a poor prognosis for a successful pregnancy.
- Fragmentation percentage: A fragment is cellular debris that is left over after a cell divides. In some embryos, when cells divide instead of dividing into two new cells, a small piece of the cell breaks off and forms a fragment. These fragments are not true cells because they lack a nucleus and cannot divide to make more cells, they are cellular “trash”. Embryos with a large amount uneven cellular pieces left over from division are graded as less likely to result in a pregnancy.
The Embryo Most Likely to Succeed…
On Day 3 all embryos are assigned a number grade, with 1 representing the highest grade, and 4 the lowest based on the characteristics above.
- Grade 1: Embryo cells are of equal size with no visible fragmentation
- Grade 2: All cells are of equal size with minor fragmentation
- Grade 2.5: Cells are mostly of equal size with moderate fragmentation
- Grade 3: Cells are of unequal size, with zero to moderate fragmentation
- Grade 4: Embryo cells are of equal or unequal sizes with moderate to heavy fragmentation
What do embryo grades mean?
Stages 1, 2, and 2.5 are the most likely to develop into the next stage on Day 5 called a blastocyst. Some stage 3 embryos will also show good potential to continue to develop to blastocyst.
Unfortunately, these visual grades are subjective and graded embryos do not always perform the way that embryologists predict they will when they are returned to the uterus.
Want to learn about Day 5 Embryo Grading? Check back next week!