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Embryo Grading

Many embryologists will tell their patients that grading is not an exact science and while they give a good indication of treatment success there are many poor quality embryos that produce live births and many good quality embryos that produce negative results.

It is important to remember that embryos that were given a “low grade” by the embryologist does not equate with a chromosomally abnormal embryo. The grading is more likely to predict pregnancy rates. In addition embryo quality, to a great extent, is determined by the quality of the egg from which it started.

There are also many different grading systems used by different clinics. It is therefore important for you to understand your clinic’s grading system before you embark on a treatment cycle. Some clinics prefer not to give their patients the embryo grading. Discuss this with your Fertility Specialist if it concerns you.

These are a few of the general criteria that the embryologist will use to grade your embryo’s.

Cell number

The cells in an embryo progressively divide up to the blastocyst stage on day 5. The cells in an embryo are called “blastomeres”. Embryos should be approximately at 2 to 4 cells at 48 hours after egg retrieval and preferably about 7 to 10 cells by 72 hours.

embryo_grading1Cell regularity – degree of regularity of size of blastomeres

It is generally better if the size of the blastomeres (cells) are similar to each other rather than there being a vast difference in sizes of cells.

This 5-cell embryo is moderately fragmented and has unevenly sized (irregular) cells. This embryo is expected to have a significantly lower chance of implanting because it is somewhat “slow” (more than 5 cells is preferred on day 3) and because of the significant fragmentation and irregularity of the cells (Image sourced from http://www.advancedfertility.com/embryoquality.htm#irregularity)

embryo_grading2Degree of fragmentation

Fragmentation, also called blebbing, is a process where portions of the embryo’s cells have broken off and are now separate from the nucleated portion of the cell. It is preferable to have as little fragmentation as possible but important to remember that some degree of fragmentation is normal in most embryo’s.

Photo of a severely fragmented and very poor quality day 3 embryo.

(Image sourced from http://www.advancedfertility.com/embryoquality.htm#irregularity)

Blastocyst Development and Blastocyst Quality Grading (or scoring)

Blastocysts (day 5 embryos) have a different grading system.

  • There are 3 distinct parts to a blastocyst which are graded – the two cell types and the fluid cavity
  • As development of blastocysts progresses, cells in the 2 components divide and the fluid cavity enlarges
  • With continued blastocyst development the embryo expands, the shell thins, and it hatches from its shell
  • Blastocyst implantation into the lining of the uterus can then begin

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