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Understanding Embryo Grading


Embryologists use a system of grading embryos to determine which are the best for transfer.  The most common scoring system was developed in the late 1990s by Dr. David Gardner and is called the Gardner System.  This system grades blastocysts starting about day 4-5 in IVF settings.

A blastocyst is an embryo that has a fluid filled cavity and is organized into both placenta (trophectoderm) and baby (inner-cell mass).


Blastocyst Development and Quality Grading (or scoring)

  • There are four distinct parts to a blastocyst – the part that forms the baby (inner cell mass), part that forms the placenta (trophectoderm), the fluid cavity (blastocoele cavity) and the shell (zona pellucida) (see figure above)

  • As development of blastocysts progresses, cells in the 2 components divide and the fluid cavity enlarges

  • With continued blastocyst development the embryo expands, the zona pellucida (ZP) thins, and it hatches from the ZP

  • Within the uterus, when the blastocyst hatches it can attach itself to the uterine lining-called implantation.

The Gardner blastocyst grading system has three separate components of quality scores:


  1. Blastocyst development stage – expansion and hatching status (number 1-6)

  2. Inner cell mass (ICM) score, or quality (letter A-D)

  3. Trophectoderm (TE) score, or quality (letter A-D)

 Blastocysts are given a quality grade for each of the three components listed above and the score is expressed with the expansion grade (how fluid filled) listed first, the inner cell mass grade listed second and the trophectoderm grade third.

For example, a blastocyst quality grade of 5AB means that the blastocyst is very expanded (grade 5), has many tightly packed cells in the inner cell mass (grade A), and has a trophectoderm with few cells forming a loose epithelium (grade B).  Examples of scored blastocytes can be seen below.

Expansion grade

Blastocyst development and stage status

1

Blastocoel cavity less than half the volume ofthe embryo

2

Blastocoel cavity more than half the volume ofthe embryo

3

Full blastocyst, cavity completely filling theembryo

4

Expanded blastocyst, cavity larger than theembryo, with thinning of the shell

5

Hatching out of the shell

6

Hatched out of the shell

Note: usually grades go to number 6 as embryos are transferred or frozen prior to hatching.

ICM grade

Inner cell mass quality

A

Many cells, tightly packed

B

Several cells, loosely grouped

C

Very few cells

 Note: Some clinics give component cells grades of D or even E in addition to the A, B and C grades described above.

TE grade

Trophectoderm quality

A

Many cells, forming a cohesive layer

B

Few cells, forming a loose epithelium

C

Very few large cells

Note: Some clinics give component cells grades of D or even E in addition to the A, B and C grades described above.

Blastocyst embryo grading is helpful and often used to select the best embryos to transfer.  However, the potential of the embryo to continue normal development, implant and result in a healthy baby born cannot be predicted on grading alone.  There may be some genetic abnormalities.  This is why many clinics will recommend the use of pre-implantation genetic testing (often called PGT-A or others).  Information on PGT-X testing is provided in other sections.

Examples of early blastocyst grading photos:






Early blastocyst                                   Early blastocyst                                    1AB

very small = Can’t yet grade              very small = Can’t yet grade      Cavity embryo’s volume = 1Inner cell mass = Can’t yet grade  Inner cell mass = Can’t yet grade      Inner cell mass = ATrophectoderm = Can’t yet grade    Trophectoderm = Can’t yet grade      Trophectoderm = B

 

Examples of intermediate blastocyst grading pictures:




4AA                                                      4AB                                                      4BB

Well expanded = 4                            Well expanded = 4                      Well expanded = 4Inner cell mass = A                           Inner cell mass = A                      Inner cell mass = BTrophectoderm = A                        Trophectoderm = B                     Trophectoderm = B

 

Examples of hatching and hatched blastocyst grading pictures (typically day 5-6):



5AA, Blastocyst hatching out of shell = 5-6 Inner cell mass = A Trophectoderm = A

Picture of a hatching blastocyst on day 6 of in vitro culture


As seen on the lower left side, it is almost hatched from the shell.  This hatching will preceed the implantation of the embryo to the uterine lining.



6AA, Blastocyst hatching out of shell = 6 Inner cell mass = A Trophectoderm = A


Completely hatched blastocyst


This embryo has a quality grade of 6AAThe tightly packed inner cell mass seen at 3 to 4 o’clock in this picture


A healthy blastocyst will implant within one to four days following a day 5 transfer, with invasion of the cells into the uterine lining occurring soon after blastocyst hatching.


Our blastocyst grading system at the Advanced Fertility Center of Chicago


  • We use a modified version of the Gardner grading system because our embryologists can not stand to be limited to only 3 grading levels for ICM and TE cells

  • The expansion grading is the same as Gardner’s

  • We use 4 grades, A-D for the ICM and TE

  • A is the same as Gardner’s A

  • B is “not quite A, but still quite good”

  • C is “not quite B, but not bad”

  • D is for very few cells and/or poor quality cells

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