By Kevin Loewke, Ph.D, Head of Data Science
After your eggs have been retrieved and fertilized, the resulting embryos will grow in culture in the lab for 3-7 days. Once they’ve reached a critical stage of growth, an embryologist will use a grading system to determine which embryo to transfer and/or freeze. To understand how this process works and why they choose certain embryos over others, it helps to start with understanding.
Immediately after fertilization, the embryo changes rapidly!
In the late 1990s, Dr. David Gardner developed and published a grading system to help embryologists determine the quality of an embryo. Based on this system, embryos are given an alphanumeric score that describes the morphological appearance (the form and structure) of an embryo. While Gardner’s system is still the most widely used, there are still several different grading systems. Your lab may have its own ‘custom’ grading system that is more or less complex than the Gardner grading system.
The Gardner grading system involves 3 quality scores:
Based on this system, a high quality embryo would have a grade of 4AA or 5AA, but an embryo still can be of good quality at a lower score. For example, there’s not much data to support which degree of expansion is best, so it’s hard to say if a 4AA embryo is better than 5AA. It’s also not clear whether the ICM or TE is more important in determining the quality of the embryo.
The other important component of embryo grading is the day an embryo reaches the blastocyst stage. If the embryo reaches blastocyst stage on day 5, rather than day 6 or 7, that’s an indication that it’s developing well. An embryo that takes 6 or 7 days to reach blastocyst stage may indicate an issue with cell division timing. However, day 6 and 7 blastocysts are still viable and capable of implanting.
Embryo grading helps embryologists and physicians understand the quality of a patient’s embryos, and decide which ones to prioritize for transfer. While a good quality embryo has a better chance of a successful outcome than a poor quality embryo, the grade alone does not guarantee a certain outcome.
It’s important to remember that manual morphology grading is also subjective. Depending on the lab and embryologist, scoring can vary. Today, many labs will perform PGT-A in addition to embryo grading to determine which embryos are euploid (chromosomally normal) and which are aneuploid (chromosomally abnormal). The standard of care for those clinics is to prioritize the transfer of euploid embryos with the best morphological grade.
Ultimately, embryo grading is a useful tool for your doctors to use to give your IVF cycle the best possible chance of success at the point of transfer.
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