Behind the Incubator Doors: How Your Embryos Are Graded and Monitored

The egg retrieval is over, and you’re back home resting. But while you are recovering, our embryology lab is buzzing with activity. For many patients, the days between the egg retrieval and the embryo transfer (or freezing) are some of the most anxiety-inducing of the entire IVF process. What exactly is happening in there?

We like to think of our embryology lab as your future baby’s first nursery. It is a highly controlled, incredibly secure environment where our skilled embryologists monitor your embryos’ growth day by day. Let’s pull back the curtain and look at how your embryos are grown, monitored, and graded.

The Timeline: Day 1 to Day 6, or Less Frequently, to Day 7

After the eggs are retrieved and fertilized (either through standard insemination or ICSI), the clock starts.

• Day 1 (The Fertilization Check): The morning after your retrieval, our embryologists check the eggs under a microscope. They are looking for two small circles called pronuclei (one from the egg, one from the sperm). If we see two, it means normal fertilization has occurred!
• Day 3 (The Cleavage Stage): By day three, the embryo is dividing. We want to see it splitting into about 6 to 8 cells.
• Days 5, 6, and occasionally 7 (The Blastocyst Stage): This is the major milestone. A blastocyst is an embryo that has divided into hundreds of cells and formed a fluid-filled cavity. At this stage, the cells begin to differentiate into two specific parts: the part that will become the baby, and the part that will become the placenta.

Making Sense of Embryo Grading

Once an embryo reaches the blastocyst stage, our embryologists give it a “grade.” You will likely hear a combination of a number and two letters (for example, a 4AA or a 3BC). While this can sound like a secret code, it’s actually a very straightforward scoring system:

1. The Number (1–6): The Expansion Score. The number tells us how expanded (or hatched) the embryo’s fluid cavity is. A 1–2 is an early blastocyst, while a 4 is a fully expanded blastocyst. A 5 means the embryo is beginning to “hatch” out of its outer shell, which it needs to do to implant in the uterus, and a 6 means it has fully hatched out.

2. The First Letter (A, B, or C): The Inner Cell Mass. The first letter grades the Inner Cell Mass (ICM) — the cluster of cells that will eventually develop into the fetus.

• A: Many cells, tightly packed.
• B: Several cells, loosely grouped.
• C: Very few cells.

3. The Second Letter (A, B, or C): The Trophectoderm. The second letter grades the Trophectoderm — the outer layer of cells that will eventually become the placenta.

• A: Many cells, forming a strong, cohesive layer.
• B: Fewer cells, forming a looser layer.
• C: Very few, sparse cells.

A Note on the “Beauty Contest”

It is completely normal to fixate on these grades and hope for “AA” embryos. But remember: embryo grading is essentially a microscopic beauty contest. It tells us how the embryo looks on the outside, but it doesn’t tell us everything about its internal genetics.

A “perfect looking” 4AA embryo might be genetically abnormal, while a “fair appearing” 3BC embryo might be normal and go on to become a beautiful, healthy baby — especially if it is tested via PGT-A screening. Our team will always guide you on the best embryo to transfer based on a combination of these grades, genetic testing results, a gender preference (if you have one), and your unique medical history.