They Gave My Embryo Some Numbers and Letters. What Do They Mean?

4AB, 3BB, “good quality”, “fair quality”… These numbers and letters used for your day-5 embryos during IVF treatment describe how the embryo looks under the microscope; they are part of an embryo grading system. So what exactly do these grades measure, and what do they tell you about the outcome of your treatment?

My patients very often ask me these questions: “Doctor, what does this grade mean?” or “Doctor, they said my embryo is fair quality. How does this affect my treatment?” If you’re ready, let’s go through them step by step.

What is embryo grading?

There is an evaluation system based on how embryos look. To explain it better, I will first talk about the structures that make up an embryo, and then give a concrete example using the 4AB grade.

What structures make up an embryo?

An embryo is a structure made up of about 100–125 cells. An embryo has

• inner cell mass: the group of cells that will form the baby,
• outer cell mass: the group of cells that will form the placenta (the baby’s afterbirth),
• blastocoel: the fluid-filled cavity inside.

The evaluation is based on how these three structures look.

What does the number in the embryo grade mean?

On day five, as the amount of fluid inside the embryo increases, the number in the grade rises from 1 to 6 accordingly. In other words, the number shows how far the fluid-filled cavity inside the embryo has expanded.

How is the inner cell mass graded?

The inner cell mass, which will form the baby, is graded as follows:

• if it consists of many cells and the cells look tightly packed — it is A quality,
• if it has many cells but they look loose — it is B quality,
• if it has few cells — it is C quality

(according to the standard grading).

How is the outer cell mass graded?

The outer cell mass, which will form the placenta, is graded as follows:

• if it consists of many bead-like, evenly lined-up cells and looks tight — it is A quality,
• if the number of cells is sufficient but it looks loose — it is B quality,
• if the number of cells is low — it is C quality

(according to the standard grading).

What does a 4AB grade mean?

Now let’s take an embryo graded 4AB as an example:

• The number 4: Shows how far the fluid-filled cavity inside the embryo has expanded, and it is a good value.
• The A in the middle: Means that the inner cell mass, which will form the baby, is of good quality.
• The B next to the A: Refers to the outer cell mass, which will form the placenta. This letter indicates that the number of cells is sufficient but the cells look slightly loose.

In summary, 4AB is considered a good-quality embryo (according to the standard grading).

Embryo grading table

Two important things to know about embryo grades

First: Although this evaluation follows a defined system, two different embryologists may give different grades to the same embryo.

Second: An embryo that looks good has a high potential to implant in the uterus. However, this does not guarantee that the embryo’s chromosome number and structure are normal. In other words, an embryo that looks good may have a high implantation potential, but this evaluation does not give definitive information about the genetic health of the embryo.

Is a 3BB embryo a bad embryo?

No. 3BB describes an embryo whose cavity is still expanding and whose inner and outer cell masses contain enough cells but look somewhat loose. Pregnancies can be achieved with these embryos too.

Can an embryo called “fair quality” be transferred?

Yes, it can. The grade is only one of the factors considered in the transfer decision; the number of embryos, age, previous treatment history and, if available, genetic test results are also taken into account.

Will a well-graded embryo always implant?

No. Although the grade is an indicator that increases the chance of implantation, it is not a guarantee on its own; the embryo’s chromosomal makeup cannot be judged from its appearance.

Does the embryo grade show the baby’s health?

No, it does not. Grading is based on how the embryo looks under the microscope; it does not give information about genetic health.

Families on the IVF journey are usually familiar with these grades. You can also share your embryo grade, whether your embryo implanted, or any experience you would like to share in the comments below.