By Kevin Loewke, Ph.D, Head of Data Science

6 Second Snapshot

• Each embryo is given a grade based on its form and structure, which helps your doctor determine which embryo has the best chance of success.
• You may also choose a more intensive form of testing called Pre-Implantation Genetic Testing to help determine which embryo to transfer.

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.

How does an embryo form?

Immediately after fertilization, the embryo changes rapidly!

• Pronuclear stage - During this stage, there are two pronuclei – one from the sperm and one from the egg – inside the embryo. Each pronuclei contains genetic material, which will combine together to form a single, 46 chromosome nucleus. When the embryologist sees these two pronuclei inside the cell, they know that fertilization has been successful!
• 2-cell stage - 1 day after retrieval, the single cell embryo will split into 2 daughter cells in a process known as ‘cleavage’ division.
• 4-cell stage - 2 days after retrieval, the 2 daughter cells will again undergo cleavage division to create a 4-cell embryo.
• 8-cell stage - 3 days after retrieval, the cells split into an 8-cell embryo. In some cases, the embryo may be transferred at this point. Usually, however, transfer occurs on day 5, 6, or 7, when the embryo has developed into a blastocyst — a rapidly dividing ball of cells.
• Morula - 4 days after retrieval, the embryo reaches the ‘morula’ stage, when cells cluster tightly together and cell boundaries become less visible.
• Early blastocyst - 4-5 days after retrieval, the embryo begins to differentiate into two different types of cells: the inner cell mass (ICM) that will eventually form the fetus, and the trophectoderm (TE) that will eventually form the placenta.
• Blastocyst - 5-7 days after retrieval, the embryo begins to fill with fluid and expand, and clearly shows the ICM and TE structures. The inner group of cells will form the embryo, while the outer group will become the cells that nourish and protect it. Today, embryo transfer usually occurs at this stage. If you’re planning to do Pre-Implantation Genetic Testing (PGT), a biopsy of the TE is taken at this stage.

How does the embryologist decide which embryo to choose?

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:

• __Expansion - this score can be 1 through 6. __
  • 1-2 - usually means poor quality, in which the blastocyst cavity forms a small portion of the embryo
  • 3 - a full blastocyst in which the cavity fills the embryo
  • 4 - the blastocyst is expanding, and the cavity is larger than the embryo
  • 5 - the embryo has begun to hatch out of the zona pellucida (shell)
  • 6 - the embryo has fully hatched (an embryo must eventually fully hatch in order to implant)
• ICM - this score can be A (best quality), B, or C (low quality)
  • A - many cells tightly packed
  • B - several cells loosely packed
  • C - very few cells
• TE - this score can be A (best quality), B, or C (low quality)
  • A - many cells forming a cohesive layer
  • B - few cells forming a loose layer
  • C - very few cells

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.

Why embryo grading matters

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.