Embryo Grading Explained: What Grade AA, AB, BB Actually Means for Success

There's an emotional part of the IVF process, and that's when you're waiting to see how your embryos are doing. You've been waiting for days through medications, scans, injections, waiting, and more waiting, and then you get a call from the fertility clinic. The embryologist tells you, "You have two AA embryos and one BB embryo."

Suddenly, the questions start. Is AA good? Is BB bad? Does this mean I have a lower chance of getting pregnant?

If you have ever asked yourself these questions, you are not alone. Because embryo grading looks like a school report card, people assume that an "A" embryo will result in a baby and a "B" embryo won't. But embryo grading doesn't work that way.

The letters are simply a tool embryologists use to describe embryos as they appear under a microscope. The grade is useful information, but it is not the whole story. No embryo is perfect, a top-graded embryo does not guarantee a pregnancy, and a less-than-perfect embryo can produce an absolutely healthy baby.

Let's look at what embryo grading actually refers to and why it is just one piece of the fertility puzzle.

What Is Embryo Grading?

Embryo grading is a technique used to assess the appearance and development of embryos during an IVF cycle.

After a sperm fertilises an egg, the egg begins to divide into more cells over the next few days. During this time, embryologists closely and carefully observe the embryos to see how they are developing.

Rather than calling an embryo "good" or "bad", they use grading criteria to describe its appearance. This helps the fertility team identify embryos suitable for transfer or freezing.

Embryo grading should be understood as an assessment of appearance. It does NOT determine whether the embryo will or will not lead to a pregnancy.

It's similar to choosing fruit at the supermarket. Two apples may look different on the outside but be just as sweet on the inside. In the same way, embryos of varying grades can lead to healthy pregnancies.

When Are Embryos Graded?

Embryos are observed throughout their development during an IVF cycle.

In the first few days after fertilisation, embryologists watch how the cells develop and divide.

Embryos are most often graded in detail at the blastocyst stage, on Day 5 or Day 6. By then the embryo has developed distinct groups of cells, which will eventually form the baby and the placenta.

At this stage, the embryologist can make a better judgement of the embryo than during its earlier development.

How Is a Blastocyst Graded?

Many clinics use a grading system based on the three most important characteristics of the embryo:

• Expansion: the size of the blastocyst, indicating how far it has grown.
• Inner cell mass: the cells that will become the baby.
• Trophectoderm: the outer layer of cells, which will form most of the placenta.

Both the inner cell mass and the trophectoderm are given a letter grade from A to C, depending on how they look under the microscope. That is why you may hear embryo grades such as AA, AB, BA and BB.

What Does an AA Grade Mean?

Generally, an "AA" embryo is a very good-looking embryo.

The inner cell mass (the future baby) is well organised and healthy, and the outer cells (the future placenta) are also strong and evenly distributed.

These characteristics are why AA embryos are a popular choice for transfer or freezing.

But it's important to understand that an AA embryo is NOT a guarantee of pregnancy. Even the best-looking embryo still has to implant well in the uterus and continue to develop.

What Does an AB Grade Mean?

An "AB" embryo also has very good potential. In this case, one part of the embryo is given an A grade and the other a B grade. For instance, the cells that will develop into the baby might look great, while the cells that will form the placenta look a little less organised. This difference is usually small, and many AB embryos result in successful pregnancies. In fact, many fertility specialists are perfectly happy to transfer embryos of this quality.

What Does a BB Grade Mean?

When people are told they have a BB embryo, they are sometimes worried about their chances of becoming pregnant. Fortunately, that worry is often unfounded. A BB embryo has a good inner cell mass and good outer cells, just not the very best. Many fertility clinics still consider these good-quality embryos, and they can certainly lead to healthy pregnancies and healthy babies. Many parents have welcomed children following the transfer of a BB-grade embryo.

Does a Better Grade Mean a Better Chance of Pregnancy?

Embryo grading provides useful information, but it is not always predictive. An embryo that appears to be in excellent condition can still fail to implant, while an average-grade embryo can implant and develop into a healthy child. This is because appearance is only one of the factors that influence an embryo's potential.

Other factors also affect the chance of pregnancy, such as:

• The quality of the eggs
• The quality of the sperm
• The health of the uterus
• Whether the embryo has the correct number of chromosomes

That is why fertility specialists do not rely on embryo grades alone.

Can Lower-Grade Embryos Lead to Healthy Babies?

Yes, absolutely. The biggest misconception about embryo grading is that lower grades are failures. In reality, there are many examples of BB (and even lower-grade) embryos that have resulted in pregnancies and healthy babies. Grading can help decide which embryos to transfer first, but it is not conclusive. Each embryo is unique, and sometimes embryos that don't look perfect under the microscope turn out to be a pleasant surprise.

Why Don't All Embryos Receive an AA Grade?

There is nothing wrong with embryos having different grades. As with all human development, embryos do not all develop at the same speed, and some will look slightly different. In a normal IVF cycle, it is common to have embryos of several different grades. Having only one AA embryo (or none at all) doesn't mean your treatment won't be successful. The goal is not to produce beautiful embryos. It is to create viable embryos that can progress to healthy pregnancies. When it comes to embryo transfer, several things help decide which embryo to use.

How Do Fertility Specialists Decide Which Embryo to Transfer?

Embryo grading is an important factor, but not the only one. The fertility team also takes into account the woman's age, her medical history, how the embryo has developed, and whether genetic testing has been performed. The results of genetic testing can be even more important than the embryo's appearance. The approach is always tailored, because every fertility journey is unique.

Should You Worry If Your Embryo Isn't AA?

Don't worry if you don't have an AA embryo! It is natural to compare embryo grades and hope for the highest one. If your embryo was graded AB or BB, don't give up hope. Embryos that don't look perfect on paper often do very well in IVF pregnancies.

Embryo grading represents the embryologist's observations at a particular point in time. It can't accurately predict the future.

Instead of reading too much into the letters, it's better to talk to your fertility specialist, who can explain what they mean for you and your treatment.

Final Thoughts

Embryo grading can seem confusing, particularly when you hear terms like AA, AB or BB in the middle of an emotional IVF journey. These letters provide valuable information about how an embryo looks under a microscope; however, they do not give a complete picture of the embryo.

AA embryos are generally excellent in appearance, but AB and BB embryos can also lead to healthy pregnancies. The grade is only one of the factors that help the fertility specialist decide which embryo to transfer first, and it is not an absolute guarantee of success or failure.

Embryo grading is an important criterion in IVF, but every pregnancy is unique and many other factors are involved. If your embryo grade is a concern, remember that these letters are just one part of a much larger picture. Understanding what they really mean, and keeping realistic expectations and hope throughout treatment, will help you make the most of it.