Embryo grading is a pivotal tool in deciding which embryos are optimal for transfer, and when to transfer them.
While each day after fertilization is a new stage of development for your embryos, grading typically takes place on the days of the most significant stages of development: day 3 and day 5.
Understanding how your embryos are graded based on their growth can help you make informed decisions about the timing of your IVF journey.
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Day 1 Embryo Grading
Embryo grading on day 1 does not take place as the embryos are placed in an incubator for 15-18 hours after fertilization. The embryos are checked, however, to see if normal fertilization has occurred.
Normal fertilization is indicated by the presence of two pronuclei, one each from the sperm and egg. If there are more or less than two pronuclei would signify a chromosomal abnormality and disqualify that embryo from use.
The embryos that are examined and determined to have been fertilized will go back into the incubator until checked again for development.
Day 2 Embryo Grading
On day two, there is no grading either as the embryos growth continues. Early cell division should be able to be observed.
Day 3 Embryo Grading [The Cleavage Stage]
Day three is known as the cleavage stage because the embryo is continuing to divide, but it is not growing in size.
By the third day of development, grading day 3 embryos is done to see if they are viable for transfer or freezing them for later use is best. Depending on when the grading is done, different characteristics are evaluated.
Cells are similar, no fragmentation is seen Cells are similar, fragmentation is minimal (0-10%) Cells are mostly similar, fragmentation is moderate (11-25%) Cells are not similar, fragmentation is moderate to severe (26-50%) Cells can be similar or not, fragmentation is moderate to heavy (>50%)Day 3 Embryo Grading Scale
Fragmentation occurs when cells divide; embryos with less fragmentation are considered higher quality. In addition to fragmentation, embryos for day three are graded on the number and appearance of cells too.
How Many Good Quality Day 3 Embryos Make It to Day 5?
Not all embryos become blastocysts. More embryos are ready for transfer on day three because attrition, or gradual loss, occurs during development. If a patient has 10 embryos after fertilization, it could become five embryos on day three and two embryos by day five.
Day 4 Embryo Grading
No grading takes place on day four but cell division picks up pace as it compacts into a morula. A morula is a cluster of cells that occurs after fertilization but before developing into a blastocyst.
Day 5 Embryo Grading [The Blastocyst Stage]
Grading day 5 embryos, also known as the blastocyst stage, is the other most common time for your medical professionals to determine which embryo is best for transfer.
A blastocyst is the early stages of an embryo; a morula that’s continued its growth, in terms of cell division and the number of cells. These cells grow into two groups that have specific roles:
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Inner Cell Mass (ICM): A ball of cells that will eventually grow into the fetus.
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Trophectoderm Epithelium (TE): A sheet of cells that will form the tissues needed during the pregnancy like the placenta.
The ICM and TE are critical for a pregnancy and graded to categorize how much the embryo has expanded. For embryos that reach the blastocyst stage, there’s a different grading system.
The most commonly used grading scale for day five embryos is the Gardner system.
Blastocyst development stage: Expansion and hatching status. ICM quality. TE quality.Gardner Blastocyst Grading System
The higher the number, the further along the blastocyst is in its development to becoming an embryo and a higher letter grade for the first and second letters indicates better quality.
What Percentage of Embryos Make It to Day 5?
On average, only 30-50% of fertilized eggs make it to the blastocyst stage. Attrition is natural as the healthier embryos make it further in the process. Age, egg quality, embryo quality and the specific IVF cycle all play a part in IVF attrition rates.
Is 4AA or 5AA Better?
Both 4AA and 5AA embryos are considered excellent when it comes to the Gardner grading system. Any embryo graded 3AA or above has clinical pregnancy rates of 65% with the difference between the numbers being an expanded blastocyst vs. a hatching blastocyst.
Day 6 Embryo Grading
Growth isn’t linear and an embryo that reaches day six simply means that the blastocyst isn’t in sync with the uterine lining.
They can still be graded but at this point, freezing the best quality embryos would be best because a low pregnancy rate of around 15% occurs for day six fresh embryo transfers. Frozen day six embryos, however, have much higher pregnancy rates of around 50%.
Day 7 Embryo Grading
Embryos can still reach the blastocyst stage seven days after egg retrieval. Similar to day six embryos, any high quality embryos will be graded and frozen for later use.
What’s Next?
The journey of an embryo from fertilization to blastocyst stage is crucial for the embryo transfer. A higher number of embryos that reach this stage can significantly improve the chances of a successful pregnancy. Higher-quality embryos, graded as "excellent" or "good," are more likely to reach the blastocyst stage and have a better chance of success.
Learn more about how many embryos make it to blastocyst.
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