A guide to embryo grading

A Guide to IVF Embryo Grading

6 min read

When going through IVF, it’s normal to hope for as many eggs as possible to be retrieved for a greater chance of producing and having multiple embryos available to transfer. But, after fertilization and blastocyst development, how do doctors select which embryo to transfer? How does a fertility clinic decide which ones have a better chance of developing into a viable pregnancy?

Here, we share how embryos are visually evaluated and what those grades are.

How are embryos graded?

An embryologist is the person who assists in fertilizing your eggs, either with IVF (where the egg is left to mingle with thousands of sperm surrounding it) or intracytoplasmic sperm injection (ICSI—where a single sperm is selected and injected directly into the egg). After the eggs are fertilized, the embryologist looks for developmental checkpoints in each embryo over the next five to seven days. This includes looking at the embryo’s cell numbers and its organization and giving the embryo a score at each milestone based on different physical characteristics. This helps determine which embryos will be best to transfer for the highest likelihood of a pregnancy.

There are three major checkpoints for scoring embryos:

Zygote Check: 16 to 18 hours post fertilization

This is the first step in your embryo’s growth. The embryologist looks at your newly fertilized eggs (now called zygotes) and takes note of the size and symmetry of the initial pronuclei (the bundles of genetic material). Not all eggs will successfully fertilize; the appearance of two pronuclei (one from the egg and one from the sperm) is the evidence embryologists look for to confirm fertilization has occurred.

Cleavage stage: Two- or three-days post fertilization

At this stage the embryos should have divided two to three times, now having at least four cells (and sometimes up to eight) that are inside an outer layer called the zona pellucida. The embryologist checks for this and looks for any abnormalities in cell division. They’ll make sure the cells are equal in size and check for any fragments in the cytoplasm (the liquid that fills the inside of the cell).

Blastocyst grading: Day five to seven

At this stage the embryos are in the last stage of development inside the IVF lab and can be used in a fresh transfer or are frozen to be transferred in a later cycle, and so they are graded to predict potential successful implantation. A blastocyst looks a lot different than a day-three embryo. There is still a zona pellucida (the shell of the embryo), which the embryo will eventually hatch out of. The outer layer of the blastocyst is called the trophectoderm, and its cells will evolve to become the placenta. There is also an inner cell mass concentrated at one end of the embryo, this group of cells with evolve into the fetus. The remaining space, or cavity, is filled with fluid called the blastocoele.

Embryo grades and what they mean

The grading of a blastocyst embryo is made up of a numerical value between one and six, followed by two letters, A through C. The highest grade is a 5AA or 6AA and the lowest grade is 1CC. 

The higher the number, the more advanced the embryo is. Each lab has its own specific criteria to determine which embryos are sufficiently advanced enough to freeze or transfer, and when to do so. For example, the IVF lab might freeze all the embryos in the morning and will therefore freeze all the appropriate embryos at the same time, halting development until they are unfrozen. (Not all embryos will reach a stage five or six before freezing—but that doesn’t mean they won’t reach a stage five or six when they thaw and evolve.) An embryo must fully hatch out of its shell (zona) before implanting into the uterus. So, to reach a positive pregnancy test an embryo must evolve into a stage six (fully hatched). On the other hand, an embryo may stop developing anywhere along its developmental journey.

The numerical digit of an embryo grade represents the degree of embryo expansion and hatching status.

  • 1: A premature embryo. The blastocoel (the space or volume of the inner cavity) is less than half the volume of the embryo, the embryo’s cells are still relatively large and taking up more space.
  • 2: A blastocyst. The blastocoele is at least half the size or more of the embryo.
  • 3: A full blastocyst. The blastocoel completely fills the embryo.
  • 4: An expanded blastocyst. The zona layer has started to thin.
  • 5: A hatching blastocyst. The trophectoderm starts pushing through the outer zona layer.
  • 6: A fully hatched blastocyst. The blastocyst has now completely emerged from the zona. 

The first letter of an embryo grade represents the quality of the inner-cell mass—the group of cells that will become the fetus.

Note: You’re hoping for an A grade here.

  • A: There are many cells, and they are tightly packed together.
  • B: There are several cells, but they’re a bit looser in their group.
  • C: There are very few cells.

Generally, a viable embryo has lots of healthy cells that continue to divide and evolve, so an A grading corresponds to a higher quality embryo.

The second letter of an embryo grade represents the grade of the trophectoderm—the group of cells that will become the placenta

Note: You’re hoping for an A grade here.

  • A: There are many cells, and they make up a proper cohesive layer.
  • B: There are several cells and a looser layer.
  • C: There are very few cells.

What is considered a good embryo grade?

It’s important to remember that embryo grading is quite subjective. Numerous studies have shown that different embryologists might visually grade the same embryos differently, and even the same embryologists might not be consistent with the grading of the same embryo when assessing a second time. Putting these visual grades into an overall classification is equally subjective, but for the purposes of better understanding, there are two studies that have used the following groupings to describe the various grades of blastocyst quality:

  • Excellent: 3AA or greater
  • Good: 3-6AB, 3-6BA, 1-2AA
  • Average: 3-6BB, 3-6AC, 3-6CA, 1-2AB, 1-2BA
  • Poor: 1-6BC, 1-6CB, 1-6CC, 1-2BB

An excellent grade embryo is typically greater than a 3AA, and these have a greater likelihood of implantation compared to poorer quality embryos.

Does having an embryo grade under 3AA mean that embryo transfer won’t be successful?

Even a lower grade embryo can successfully implant and lead to a positive pregnancy and healthy live birth. Keep in mind that these are all developmental stages, and a particular embryo may just need more time to keep growing and developing. It’s best to think of embryo quality as a spectrum—although higher-graded embryos have a higher chance of success, it doesn’t mean a low-quality embryo can’t be successful.

Why do we rely on visual embryo grading?

Embryologists have to select an embryo to transfer, so they inherently have to rely on some evaluation to rank them. Grading embryos visually is non-invasive information that can be gathered about embryo quality just by looking, without having to remove any cells. Embryos are graded so that your doctor can choose the most promising one that might lead to a live birth—based on the embryo’s developmental journey. This also helps reduce the need to transfer more than one embryo at a time, therefore reducing the likelihood of multiples in a pregnancy. Having more insights into the quality of the embryo being transferred can be an important counseling tool as well. If an embryo transfer cycle failed and the embryo grading was poor, then most likely the embryo that was transferred wasn’t viable, as opposed to other potential contributing factors (for example, endometrial thickness, thyroid function, etc.).

Will fertility clinics transfer embryos regardless of grading?

If you only have lower-scored embryos available, your doctor may still suggest transferring them—but this depends on your unique situation and factors including your age, egg reserve and how well you responded to your egg retrieval medications. In some cases, your doctor may suggest doing another egg retrieval to try to improve the yield of eggs and quality of embryos. You may also wish to take a short break and work on improving egg quality with preconception healthcare (like making recommended lifestyle changes or taking additional supplements) before another retrieval. In other cases, it may be worth trying to transfer a lower grade embryo (especially if the toll—financially, physically and emotionally—of another egg retrieval is too much for you). Your fertility doctor can walk you through the options and suggest the best course of action. 

Remember that ultimately these are your embryos, and your fertility clinic is there to guide you on your options.

 Feature Image: Future Fertility