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19 January 2024

Decoding Embryo Grading: A vital aspect of an IVF Journey

When couples embark on an IVF journey to overcome fertility challenges, one critical aspect they come across is ‘embryo grading’.

 The process of assessing and grading embryos is an important step in determining which embryos are most likely to result in a successful pregnancy. In this blog, we will explore the importance of embryo grading, how it is done, and why it is a key factor in the IVF process. Samantha Knight, Embryology Laboratory Manager at Harley Street Fertility Clinic, helps us to understand the mysteries of embryo grading.


The Significance of Embryo Grading

Embryo grading determines the quality and development of embryos created during IVF or ICSI procedures. Fertility consultants and embryologists can then select the embryos most likely to increase the chances of a successful pregnancy.


Key aspects of embryo grading


Morphological assessment

When assessing the developmental stage of the embryos, embryologists examine the appearance and characteristics of the embryos, looking for specific features such as:

  • The number and symmetry of cells: A healthy embryo undergoes cleavage divisions and a balanced distribution of cells. Irregularities in cell number or uneven distribution may indicate developmental issues.
  • The degree of cell fragmentation: the embryo can have small, membrane-bound fragments. Excessive fragmentation can indicate poor embryo quality. Embryologists evaluate the extent of fragmentation as it can impact the embryo's potential for successful implantation.
  • The size and shape of the blastomers (individual cells): Blastomeres should be relatively uniform. Inconsistencies in size or irregular shapes can indicate problems in the embryo's developmental health
  • The appearance of the outer shell of the embryo: A healthy embryo typically has a clear and appropriately thick outer shell (zona pellucida). Anomalies can affect the embryo's ability to hatch during implantation.


The Blastocyst and the grading system

The blastocyst is an embryo in the early developmental stages. It is typically formed around 5 days after fertilization, and it consists of two main parts: the inner cell mass and the trophectoderm. The inner cell mass will form the foetus, while the trophectoderm will form the placenta.

A widely-used grading system is the Gardner grading system, which assigns embryos a grade based on their appearance and development. Using a number and 2 letters A-C, the embryos are then graded.


The significance of letters and numbers

Samantha explains,

  • The number indicates the stage (day) of the development.
  • The first letter indicates the overall development of the inner cell mass cell. A, B or C show good overall development.
  • The second letter indicates the quality of the


For instance, a high-quality embryo might receive a grade of 4AA, indicating that it has reached the blastocyst stage (day 4), with well-defined inner cell mass and trophectoderm. Other examples of embryo grading:

  • 2BB: This could indicate an embryo at an earlier developmental stage (day two) with moderately well-defined inner cell mass and trophectoderm.
  • 3BC: An embryo at a slightly later stage (day three) with good overall development but with a lower quality trophectoderm compared to higher grades.

Other combinations, such as AB or BA, may indicate that one of the components is of slightly lower quality than the other. The specific grading system used and the criteria for assigning letters can vary among fertility clinics and embryologists.


Selecting the best embryos

The primary goal of embryo grading is to identify the embryos with the highest potential for successful implantation and pregnancy. Embryologists and clinicians use these grades to make informed decisions about which embryos to transfer or freeze for future use. Higher-graded embryos are more likely to result in a successful pregnancy.


Embryos’ gradings- no guarantee of success

Although the embryo grading is informative, is not a guarantee of pregnancy success. It's just one piece of the puzzle. Several other factors are involved , including the patient's age, reproductive history, and overall health, also play a role in the success of assisted reproductive procedures. Additionally, embryos that receive lower grades may still lead to healthy pregnancies, so the grading is not absolute.


Advanced knowledge through AI

At the forefront of technological advancements in assisted reproduction, Harley Street Fertility Clinic has embraced the cutting-edge AI technology known as Chloe EQ™ (Cultivating Human Life Optimising Embryos). In partnership with Fairtility, we proudly extend this innovative tool to all our patients undergoing IVF or ICSI. Chloe EQ™, coupled with our state-of-the-art time-lapse incubator, revolutionises the process of embryo selection. By simplifying intricate data and offering data-led advanced insights into early embryo development, Chloe EQ™ empowers our experienced team of embryologists to make more informed decisions, helping patients in their path to parenthood.



Embryo grading is an important element of a fertility journey, offering valuable insights into the quality and developmental stage of embryos. While high-quality embryos are more likely to result in a successful pregnancy, the process is not guaranteed - a comprehensive approach is needed, including considerations of uterine health, immunology etc. Patients should work closely with their clinic to understand the grading process and its implications for their specific situation. Further information on other elements of IVF can be discovered on our treatments pages.