Embryos created during in vitro fertilization (IVF) are graded to determine which ones are fit for transferring. This is done to choose which embryo has the highest chance of implant in the uterus and then give rise to the birth of a baby.

When discussing embryo morphology, healthcare practitioners are referring to the embryo’s appearance under a microscope. Each fertility clinic has its own scoring system to distinguish between poor, average, and good quality embryos.

Although embryo scoring is not an exact science, it is a useful tool to determine which embryos may be of better quality. Nevertheless, there are several other factors that determine if an embryo has the capacity to develop into a baby, and many of these factors can’t be deduced just by looking at the embryo through a microscope.

Each fertility lab may have different approaches to grade the embryo’s morphology; however, the common characteristics they evaluate are the embryo’s inner cell mass (ICM), trophectoderm, and degree of expansion.

The inner cell mass refers to the little cluster of cells on the interior of the embryo, which will later develop into the actual fetus.

The trophectoderm are the cells that surround the embryo and which will later become the placenta.

Some clinics grade the embryos on day 3 of growth (cleavage stage); in this stage, a grading system that rates the embryos on a scale of 1 to 5 is typically used. Usually, a score of 1 is better; but, some clinics may use 5 as the best score.

At this stage of development, day 3 embryos are graded based on the rate of growth and the degree of cell fragmentation.

Fragmentation occurs when parts of the cell break off during cell division. This is a common occurrence, and healthy babies can be born from embryos that presented fragmentation at this stage. However, embryos with more than 25% fragmentation may have a lower implantation potential.

Ideally, day 3 embryos have at least 6 cells and preferably 8 cells, surrounded by an outer shell called the zona pellucida.

On day 5, the embryos should have grown into blastocysts. At this stage, the embryo has two kinds of cells: the cells that will develop into the baby (inner cell mass) and the cells that will generate the placenta (trophectoderm), surrounding a fluid-filled cavity. Blastocyst scoring is determined by:

• Appearance of the inner cell mass which is scored with either an A, B, or C grade. A being the best score.

• Appearance of the trophectoderm also scored with A, B, or C grade. A being the best score.

• Degree of expansion, based on how expanded the fluid-filled cavity is. The degree of expansion is scored on a scale of 2 to 6, with 6 being the most expanded.

The best characteristics an embryo could have include:

• Large degree of expansion

• Uniformly-sized cells in the trophectoderm

• A thick-looking inner cell mass with many cells tightly packed together

The final score assigned to an embryo is based on those three characteristics. Each embryo is given two letter scores, the first one for the inner cell mass appearance and the second one for the trophectoderm appearance, along with a description of the degree of expansion, which is scored with a number from 2 to 6. The letter grades are A, B, or C, with A being the best.

This scoring system gives room for a lot of variation, and since the score is based on what the healthcare practitioner sees on the microscope, the final score can vary between fertility clinics.

Based on the degree of expansion of the embryo’s cavity, the embryo score could be:

• 2: the cavity fills 1/3 of the embryo

• 3: the expansion is partial, filling 70% of the embryo

• 4: a fully expanded cavity

• 5: the embryo has expanded and split open the zona pellucida

• 6: the embryo has completely hatched from the zona pellucida

Based on the inner cell mass, the embryo score could be:

• A: many cells, tightly packed and well defined

• B: several cells, loosely grouped, may appear as grainy

• C: very few dark cells

Based on the trophectoderm, the embryo score could be:

• A: many smooth equally-sized cells, forming a neat layer

• B: a few irregular grainy cells, forming a loose epithelium

• C: very few large cells, which may be dark or grainy

In this scoring method, an embryo with a final score of 4AA would be a blastocyst of very good appearance (a fully expanded cavity, many cells tightly packed and well defined in the inner cell mass, and a trophectoderm with many smooth equally-sized cells that form a neat layer).

However, there is no need for an embryo to have a perfect AA score in order to result in a baby. Most babies, in fact, come from grade BB and BC embryos.

It is important to notice that an embryo score only indicates its potential to implant in the uterus successfully and has no connection to how beautiful or intelligent your baby will be.