What is Embryo Grading?

One of the most exciting moments in the IVF process is learning that you have viable embryos. Before your embryo transfer procedure, you’ll need to decide which embryo(s) to implant. To help with this, your embryos will be graded to narrow down the likeliest candidates for successful implantation.  

This sounds simple enough, but there’s a lot of information that goes into understanding how embryos are graded. Below is a break down of everything you need to know in order to understand your embryos’ report cards. 

What are the stages of embryo development?  

Understanding the stages of an embryo’s development is the first step towards understanding embryo grading. Once your egg is successfully fertilized, it will need to pass through the following stages before it’s mature enough for implantation: 

• Zygote: This is stage one. An egg and sperm have met, and fertilization has occurred. 
• Compaction: At this stage, an eight-cell embryo undergoes a series of changes, transforming the zygote into a solid ball of cells. 
• Morula: Resembling a blackberry, at this stage the embryo has formed into a solid mass of 12 to 15 cells. This normally happens around four days after fertilization. 
• Blastocyst: Five days after fertilization, blastocyst formation begins. Blastocysts have a round shell, a fluid-filled cavity, and an internal cell mass that will become the fetus. 
• Hatched Blastocyst: Around six days after fertilization, the blastocyst hatches from its protective shell in order to implant in the uterine wall. If an embryo reaches this stage in the lab, it is no longer viable for embryo transfer. 

Once your eggs are retrieved and fertilized, an embryologist will watch them closely for the next three to five days to ensure your embryos are developing accordingly. 

What is embryo grading?  

One of the biggest upsides to in vitro fertilization (IVF) is the amount of control you have over every step of the process. This extends to choosing which embryo(s) to implant. Your fertility specialist won’t just pick randomly and then hope for the best. Instead, they’ll use the Gardner Embryo/Blastocyst Grading System to evaluate each embryo’s quality.  

In broad strokes, this system evaluates different aspects of your embryos’ development to give you a clear understanding of which will be the best candidate(s) for a successful implantation. Knowing this will allow you to make an informed decision about which embryos to implant, which to freeze or donate, and which should be discarded. 

How are embryos graded?  

The Gardner Embryo/Blastocyst Grading System is what embryologists use to determine embryo development and, ultimately, the potential each embryo has for successful implantation. More specifically, the embryo grade system looks at the development in two stages: 

• The day-three embryo, or “cleavage stage” embryo, is graded based on the number of cells and their structure. 
• The day-five embryo, or blastocyst, is graded based on a scale of blastocyst development, the quality of the inner cell mass (which will become the fetus), and the quality of the trophectoderm (which are the outer cells that will become the placenta and other essential tissues). 

At both stages, these rubrics will help narrow down which embryos are ideal for transferring or freezing for future use. 

Day 3 embryo grading  

The third day of embryo development is the first time your embryos will be graded. At this stage, the embryo will have undergone compaction, and the cells will start to divide. Embryologists call this the “cleavage” stage. 

To grade your embryos, the embryologist uses a high-powered microscope to look at their overall structure, or morphology, and count how many cells there are. 

Cell appearance  

Cell appearance is often more challenging to grade than cell number. Embryologists have to account for factors such as whether the cells are all of equal size, whether each cell has a nucleus, and how much fragmentation is present as a result of cell division. All of these factors contribute to the quality of your embryos. 

Day-three embryos are graded on a scale of one to four, with one being the highest grade. This aspect of embryo grading can be subjective, as the embryologist has to interpret what the embryo’s appearance says about its quality. Therefore, it is possible for two embryologists to grade the same embryo differently. 

Cell number 

Grading embryo cell numbers is more clear-cut. By day three, an embryo typically has between six and ten cells. Research shows that a minimum of eight cells is ideal, as day-three embryos with eight or more cells show much higher live birth rates. 

Day 5 embryo grading (blastocyst)  

If you decide not to do a day-three embryo transfer, your embryos will be graded a second time on the fifth day after fertilization. At this point, your embryos will have reached the blastocyst stage and will be nearing the final stage of development. 

Grading blastocysts is more complex than grading day-three embryos because your embryologist is looking at a very different set of factors. When looking at your embryo grades, it’s important to remember that because of this complexity, the grades may change as the embryos continue to develop. 

The following are the three main aspects embryologists evaluate: 

• How much the blastocyst has expanded: This means measuring the blastocoel, which is the fluid-filled cavity that forms within the embryo. 
• The quality of the inner cell mass (ICM): These inner cells will eventually form the fetus if implantation occurs. 
• The quality of the trophectoderm epithelium (TE): These outer cells form the placenta and other essential tissues if implantation occurs. 

When you receive your embryo grades, they will each have a number to indicate blastocyst expansion, along with two letters indicating ICM and TE quality. In the next section, we will break down what each grade means. 

Day 5 embryo grading charts explained  

As mentioned above, your blastocyst grades have three components: a number for blastocyst growth and letters for ICM and TE quality. An example grade would be 5AA, which is among the highest quality possible. We’ve broken down blastocyst grading into tables to help you understand your embryo grades. 

The number: Blastocyst expansion 

Unlike with day-three embryos, the numbers are ascending, with five being the highest grade and one the lowest. 

Grade of expansion	Name	Meaning
1	Early Blastocyst	The blastocoel cavity is less than half the volume of the embryo.
2	Blastocyst	The blastocoel cavity is greater than half the volume of the embryo.
3	Blastocyst	The blastocoel cavity fills the embryo.
4	Expanding	The blastocoel cavity is greater than the embryo, and the protective membrane has thinned.
5	Hatching	The TE is starting to burst through the protective membrane.
6	Hatched	The TE has fully burst through the protective membrane.

It should be noted that a fully hatched blastocyst (a grade of six) is no longer viable for embryo transfer. 

The first letter: Inner cell mass quality 

With the letter grade, it works much the same as on a regular report card, with A being the highest grade. 

Grade	Description
A	The ICM is tightly packed and contains many cells.
B	The ICM is loosely grouped and contains several cells.
C	The ICM contains very few cells.

The second letter: Trophectoderm quality 

Grade	Description
A	The TE consists of many cells, forming a tightly knit epithelium.
B	The TE consists of a few cells.
C	The TE consists of very few cells, forming a loose epithelium.

How does embryo grading impact IVF success rates?  

This is the tricky part. Researchers are still exploring what role embryo quality plays in IVF success rates. Some studies indicate that better-quality embryos have higher pregnancy rates and more live births. However, that doesn’t necessarily mean lower-quality embryos won’t result in a successful IVF cycle. 

There is ample evidence that transferring lower-quality embryos can still result in pregnancy and live birth. All this to say, don’t be concerned if your embryo(s) haven’t achieved a 5AA grade. 

When it comes to freezing embryos, doctors prefer to preserve higher-quality embryos. This is something to keep in mind if you opt for a frozen embryo transfer. 

Embryo grading alone won’t predict whether or not your IVF treatment is going to be successful, so it’s important not to get too excited or discouraged by the results. It’s one factor among many that contribute to a successful embryo implantation process. This is a good time to consider pre-implantation genetic testing (PGT) to verify the genetic health of the embryos you are considering transferring. 

When should you schedule your embryo transfer? (Day 3 vs. Day 5)  

There are two schools of thought when it comes to deciding when to undergo your embryo transfer. However, there is currently no consensus on which method is better, and each carries its own benefits and risks. 

On the one hand, it’s believed that transferring embryos on day three will increase the likelihood that they’ll reach a healthy blastocyst stage in utero. Some fertility specialists prefer to transfer multiple embryos at day three. This will, of course, increase your risk of having multiple births. 

On the other hand, transferring embryos in the blastocyst stage is believed to reduce the need to implant multiple embryos, as blastocysts are more primed for implantation. There’s also evidence that preimplantation genetic testing is more sensitive at day five.  

More fertility clinics are moving toward day-five embryo transfers. Your fertility specialist will weigh your medical history and risk factors, as well as the quality of your embryos, before deciding when to schedule your embryo implantation.