What Is Embryo Grading in IVF?

Babies who are born with the help of assisted reproductive technologies, such as IVF, undergo further growth and development stages through their embryonic phase in the lab. The embryologists consider different grading systems for the embryos before they are transferred to the mother's uterus. One of the important stages in the IVF process is examining the embryo's quality in a laboratory to select a better-quality one for transferring.

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In other words, embryo quality or grading system is one of the effective factors in a successful IVF treatment. Since the embryo is usually transferred to the uterus on day 3 (cleavage stage) or day 5 (blastocyst stage), it is essential to examine some embryo characteristics such as inner cell mass, Trophectoderm, and the degree of blastocyst expansion to detect a healthy embryo.

Embryo Grading in IVF

What Is Examined in Embryo Grading System?

The embryologists use the following terms to examine the embryo characteristics:

Morphology: The term morphology refers to the form and structure of an embryo. Examination of embryo characteristics is often called morphology evaluation by specialists.

Morula: A few days after fertilization, and when the embryo goes through several cell division phases, the cells enter the blastocyst forming stage in which their borders cannot be distinguished from each other. This stage, called a morula, usually occurs between days 3 and 4 after fertilization.

Blastocyst: The embryo enters the blastocyst stage about 5 and 6 days after fertilization and cell division. A blastocyst is an embryo development stage in which the inner cell mass, Trophectoderm (surface cells), and the blastocoel cavity are formed.

Inner cell mass: The inner cell mass (ICM) is a group of cells inside the blastocyst that will eventually become a real embryo.

Trophectoderm refers to the cells surrounding the blastocyst's outer layer and is involved in embryo implantation. These cells eventually become the placenta.

embryo grading

Grading the Embryo Quality in IVF

In the IVF method, the first report of the pregnancy status is given to the patient one day after egg retrieval, followed by laboratory fertilization. However, evaluating the embryo's quality accurately at this stage is impossible. Therefore, the embryo grading method is used to detect high-quality embryos. 

The embryos derived from in vitro fertilization are kept in an incubator. As embryos need to be in a stable environment, the number of times they can be put out of the device is limited.

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Determining Embryo Quality in IVF

Generally, the embryo's quality is determined on day 3 or 5 after egg retrieval and laboratory fertilization. The reason is that until day 3, most of the embryo's growth and development relates to the egg contents. From the end of day three onwards, the sperm genetics enters the embryo's DNA, and the embryo genetics becomes complete and active. On day 5, embryo grading is done by measuring blastocyst expansion, Trophectoderm, and the inner cell mass. The following describes the embryo quality or grading on days 3 and 5 after fertilization.

Grading the Embryo Quality in IVF

Grading System of Day 3 Embryo

The number of cells increases in a day three embryo due to proliferation; however, the embryo size does not change. Embryo grading is done after three days for the first time. For this purpose, embryologists examine the embryo morphology with a high-power microscope, during which they evaluate both cell shapes and numbers in the embryo:

The shape of embryo cells: It is difficult to grade their morphology and appearance on day 3; therefore, the cells are divided into 4-5 groups. We should also mention that the embryo is graded by the fragmentation degree at this stage. A fragmentation percentage of up to 20% is acceptable, but if it goes higher, it will affect the function of the cytoplasm.

In general, a day three embryo appearance is examined according to the following aspects: 

  • Each cell to be mononuclear;
  • Cells sizes to be appropriate and uniform;
  • Cell division and cell content to be well done for evaluating the optimal function of the cytoplasm;
  • Adequate density and compression of cells and their readiness to form blastocyst;
  • The presence of a cytoplasmic cavity in the cell contents;
  • The proper condition of vacuoles.

The Number of Embryo Cells: One of the essential indicators to determine a healthy and high-quality embryo is the number of cells in a day three embryo. Thus, according to a general rule, an embryo that's dividing well should have between 6 to 10 cells, ideally eight cells, by day 3. However, this number may sometimes reach 3, 5, or 6 due to non-synchronous cell division.

How to Grade the Quality of a Day 5 Embryo

Embryo grading usually follows a universal rule. However, each fertility clinic may have a unique grading system with a little different from others, which causes a slight difference in the embryo grading results. According to references, determining and evaluating a day five embryo grade is more complicated than a day 3 one.

The embryologists examine the following aspects at this stage:

Degree of blastocyst expansion: The amount that the blastocyst expands is called the blastocyst expansion degree. In this stage, the size of the blastocoel (fluid-filled cavity in the embryo) is measured, which varies between 1 and 6. The best size is 6 in this stage. A day five embryo grading in the blastocyst stage is done as the following:

  • Number 1: Blastocoel cavity has filled less than half of the embryo.
  • Number 2: Blastocoel cavity has filled more than half of the embryo.
  • Number 3: Blastocoel cavity has filled 70% of the embryo, and partial expansion has occurred.
  • Number 4: Blastocoel cavity is fully expanded.
  • Number 5: Embryo has expanded, hatching occurs, and the zona pellucida is split open,
  • Number 6: Embryo has completely hatched from the zona.

How to Grade the Quality of a Day 5 Embryo

Inner cell mass quality: The embryos are classified from grade A to C during the inner cell mass (ICM) quality examination, with A being the best.

  • Grade A: Embryo cells are uninformed.
  • Grade B: Embryo cells are less uniform with minor fragmentation. They may look granular in appearance.
  • Grade C: Cells are dark and low in quantity with major fragmentation.

Trophectoderm Quality: The trophectoderm quality is determined by forming the placenta and other pregnancy tissues. Like the ICM, the Trophectoderm is classified as grades A to C. Grade A embryos have the best quality:

  • Grade A: A solid layer of many flat, equal-sized cells is formed.
  • Grade B: The outer cell layer is uneven, and some cells look granular.
  • Grade C: The embryo cells are dark or look granular, and the outer cell layer is very uneven.

How to Report Embryo Grading in IVF

In reporting embryo grading with two letters, the first letter indicates the ICM degree, and the second indicates the trophectoderm degree. For example, grade AB indicates that the embryo has a high-quality inner cell mass (ICM) with grade A and a good trophectoderm (cells surrounding the blastocyst) with grade B.

Appropriate Grade for Embryo Freezing

Only high-quality embryos, which will bring a healthy pregnancy, are frozen in fertility clinics. However, this does not mean that a perfect embryo with a grade of AA is needed to get pregnant since most embryos born with healthy babies were graded as BB and BC. Even sometimes, ICM may be graded as C and change to grade A over time. The blastocoel expansion degree with number 1 can also turn into number 6 within one day.

frozen embryo transfer
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Does Embryo's Grade Affect IVF Success Rate?

The purpose of assisted reproductive methods such as IVF is to increase the chances of a successful pregnancy and giving birth to a healthy baby. Therefore, specialists ask for important information, such as the patient's age, fertility history, etc., and then use the embryo grading method to identify high-quality embryos. However, we should note that according to research and scientific findings, embryo grading is not a very accurate method and is used as an auxiliary method.

It should also be considered that the embryo grading system only examines and evaluates the embryo's ability to be implanted. It is not a criterion for the baby's intelligence or cuteness.

ivf embryo grading


Embryo grading is one of the important stages of IVF. As mentioned, information such as the mother's age, her previous pregnancy experiences, the number of embryos that can be transferred, and a suitable day for transferring are checked in the embryo grading system so that a good embryo will be selected to increase the chance of a successful pregnancy.

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