Cati
The Science

Day‑Five Embryos Grading System

#
Introduction

Senior embryologists behind the CATI tool developed a smart day‑five embryos grading system which defines rules based on the timing of morphokinetic markers. Grading means sorting the embryos in correlation with their implantation potential (IP). The IP of embryos in our grading system is suggested based on the assumption that the embryos are ideally transferred into an ideal in-vivo environment.

Grade Part Description
1
A
A

The 1st number
Stage of Development
The Number in the first position of the CATI grading system represents a degree of embryo's blastocyst cavity and its progress in hatching out of the zona pellucida on the scale from 1 to 8.
1
A
A
 Hatching Blastocyst
2
A
A
 Expanded Blastocyst
3
A
A
 Expanding Blastocyst
4
A
A
 Blastocyst
5
A
A
 Early Blastocyst
6
A
A
 Morula
7
D
 
 Arrested Embryo
8
8
 
 Unfertilized
1
A
A

The 2nd letter
Quality of Cleavage
Second position in the grading system represents the quality of an embryo. Values (A, B, C, D) are assigned according to observations of cell division occurring within the embryo.
1
A
A
 A is normal development with linear expansion
1
B
A
 B is development with collapse
1
C
A
 C means low number of trophectoderm cells
1
D
A
 D means degenerated
1
A
A

The 3rd letter
Event Timing
The 3rd refers to timing and count of events — expansion, collapse — that are typical for the stage of development and the quality of cleavage.

For normal development (A):
1‑5
A
A
 delayed or early onset of expansion
1‑5
A
B
 middle onset of expansion
1‑5
A
C
 late onset of expansion
6
A
A
 delayed compaction of blastomeres

For development with collapse (B):
1‑4
B
A
 one early collapse
1‑4
B
B
 one late collapse
1‑4
B
C
 multiple collapses

For low number of trophectoderm cells (C):
1‑6
C
A
 low count of trophectoderm cells

For degenerated (D):
1‑8
D
A
 degenerated (no significant event observed)
1
C
A
Low
The Likelihood of Successful Implantation
An expression of the embryo's capability to achieve successful implantation.

The values are:
  • Very High
  • High
  • Moderate
  • Low
  • Very Low
  • No Chance
List of grades

List of Grades

You are already familiar with the grading system's basics. Below, you will find all defined grades together with time‑lapse embryos examples and morphokinetic rules.

Hatching Blastocyst

1
A
A
Very High
Embryo #7087 Show on time‑lapse

Hatching blastocyst with early onset of expansion

Hatching blastocyst starting expansion within 100 hrs after fertilization.
Expansion is linear without any collapse/re-expansions.

1
A
B
Very High
Embryo #8397 Show on time‑lapse

Hatching blastocyst with middle onset of expansion

Hatching blastocyst starting expansion after 100 hrs from fertilization.
Expansion is linear without any collapse/re-expansions.

Hatching blastocyst with early collapse

Hatching blastocyst starting expansion within 100 hrs after fertilization.
Expansion is interrupted by one collapse of blastocoel occurring within 105 hrs after fertilization.

Hatching blastocyst with late collapse

Hatching blastocyst starting expansion after 100 hrs after fertilization.
Expansion is interrupted by one collapse of blastocoel occurring after 105 hrs after fertilization.

1
B
C
Moderate
Embryo #8613 Show on time‑lapse

Hatching blastocyst with multiple collapses

Hatching blastocyst starting expansion within 110 hrs after fertilization.
Expansion is interrupted by multiple collapses of blastocoel.

1
C
A
Moderate
Embryo #793 Show on time‑lapse

Hatching blastocyst with low number of trophectoderm cells

This embryo has few TE cells, forming a loose epithelium and in extreme situation TE is formed by very few large cells.
Many of the abnormally cleaved cells are excluded from the embryo into the PVS (periviteline space) and are squeezed under the ZP by growing blastocyst.
One or more collapses of blastocoel can be detected.

1
D
A
Very Low
Embryo #1455 Show on time‑lapse

Hatching blastocyst with degeneration/vacuolization of majority of the cells

Hatching blastocyst with total collapse of blastocoel cavity without re-expansion.
The blastomeres undergo degeneration that is expressed by the total cell lysis or extreme vacuolization.
No further progress in development is achieved.
One or more collapses of blastocoel can be detected before total collapse.

Expanded Blastocyst

2
A
A
Very High
Embryo #10790 Show on time‑lapse

Expanded blastocyst with early onset of expansion

Expanded blastocyst starting expansion within 100 hrs after fertilization.
Expansion is linear without any collapse/re-expansions.

2
A
B
Very High
Embryo #4743 Show on time‑lapse

Expanded blastocyst with middle onset of expansion

Expanded blastocyst starting expansion from 100–105 hrs after fertilization.
Expansion is linear without any collapse/re-expansions.

2
A
C
Very High
Embryo #15010 Show on time‑lapse

Expanded blastocyst with late onset of expansion

Expanded blastocyst starting expansion after 105 hrs after fertilization.
Expansion is linear without any collapse/re-expansions.

Expanded blastocyst with early collapse

Expanded blastocyst starting expansion within 100 hrs after fertilization.
Expansion is interrupted by one collapse of blastocoel occurring 100–105 hrs after fertilization.

Expanded blastocyst with late collapse

Expanded blastocyst starting expansion within 105 hrs after fertilization.
Expansion is interrupted by one collapse of blastocoel occurring 105–110 hrs after fertilization.

2
B
C
Moderate
Embryo #5109 Show on time‑lapse

Expanded blastocyst with multiple collapses

Expanded blastocyst starting expansion within 105 hrs after fertilization.
Expansion is interrupted by 2 and more collapses of blastocoel.

2
C
A
Moderate
Embryo #10409 Show on time‑lapse

Expanded blastocyst with low number of trophectoderm cells

This embryo has few TE cells, forming a loose epithelium and in extreme situations, TE is formed by very few large cells.
Many of the abnormally cleaved cells are excluded from the embryo into the PVS (peri viteline space) and are squeezed under the ZP by growing blastocyst.
Expansion is interrupted by one or more collapses of blastocoel.

2
D
A
Very Low
Embryo #10633 Show on time‑lapse

Expanded blastocyst with degeneration/vacuolization of majority of the cells

Expanded blastocyst with total collapse of blastocoel cavity without re-expansion.
The blastomeres undergo degeneration that is expressed by the total cell lysis or extreme vacuolization.
No further progress in development is achieved.
One or more collapses of blastocoel can be detected before total collapse.

Expanding Blastocyst

Expanding blastocyst

Expanding blastocyst starting expansion within 105 hrs after fertilization.
The volume of the blastoceol cavity increased the diameter of the embryo and thinned the zona pellucida.
Expansion is linear without any collapse/re-expansions.

3
B
A
Moderate
Embryo #7572 Show on time‑lapse

Expanding blastocyst with one or more collapses

Expanding blastocyst starting expansion after 105 hrs after fertilization.
The volume of the blastoceol cavity increased the diameter of the embryo and thinned the zona pellucida.
Expansion is interrupted by one or more collapses of blastocoel.

3
C
A
Moderate
Embryo #4258 Show on time‑lapse

Expanding blastocyst with low number of trophectoderm cells

This embryo has few TE cells, forming a loose epithelium and in extreme situation TE is formed by very few large cells.
Many of the abnormally cleaved cells are excluded from the embryo into the PVS (peri viteline space) and are squeezed under the ZP by growing blastocyst.
Expansion is interrupted by one or more collapses of blastocoel.

3
D
A
Very Low
Embryo #7362 Show on time‑lapse

Expanding blastocyst with degeneration/vacuolization of the majority of cells

Expanding blastocyst with total collapse of blastocoel cavity without re-expansion.
The blastomeres undergo degeneration that is expressed by the total cell lysis or extreme vacuolization.
No further progress in development is achieved.
One or more collapses of blastocoel can be detected before total collapse.

Blastocyst

4
A
A
Moderate
Embryo #4000 Show on time‑lapse

Blastocyst

Blastocyst starting expansion after 105 hrs after fertilization.
Blastocoel cavity completely fills the embryo.
No periviteline space is visible.
The blastocyst volume still did not increase the diameter of the embryo to the extendt needed to thin the zona pellucida.
Expansion is linear without any collapse/re-expansions.

Blastocyst with one or more collapses

Blastocyst starting expansion after 105 hrs after fertilization.
Blastocoel cavity completely fills the embryo.
No periviteline space is visible.
The blastocyst volume still did not increase the diameter of the embryo to the extent needed to thin the zona pellucida.
Expansion is interrupted by one or more collapses of blastocoel.

Blastocyst with low number of trophectoderm cells

TE is formed by very few large cells.
Many of the abnormally cleaved cells are excluded from the embryo into the PVS (peri viteline space) and are squeezed under the ZP by growing blastocyst.
One or more collapses of blastocoel can be detected.

4
D
A
Very Low
Embryo #6870 Show on time‑lapse

Blastocyst with degeneration/vacuolization of the majority of cells

Blastocyst with total collapse of blastocoel cavity without re-expansion.
The blastomeres undergo degeneration that is expressed by the total cell lysis or extreme vacuolization.
No further progress in development is achieved.
One or more collapses of blastocoel can be detected before total collapse.

Early Blastocyst

Early blastocyst

Early blastocyst with visible blastocoel cavity formation.
The cavity fills less than half volume of the embryo.
Perivitelline space (PVS) is still present and no degenerated cells are found in the PVS.

Early blastocyst with low number of cells

Early blastocyst with visible blastocoel cavity having low number of cells.
Many of the abnormally cleaved cells are excluded from the embryo into the PVS (periviteline space).
Expansion is interrupted by one or more collapses of blastocoel.

5
D
A
Very Low
Embryo #5838 Show on time‑lapse

Early blastocyst with degeneration/vacuolization of the majority of cells

Early blastocyst with total collapse of blastocoel cavity without re-expansion.
The blastomeres undergo degeneration that is expressed by the total cell lysis or extreme vacuolization.
No further progress in development is achieved.
One or more collapses of blastocoel can be detected before total collapse.

Morula

Morula

No blastocoel cavity is visible.
No degenerated cells are found in the PVS (perivitelline space).

6
C
A
Very Low
Embryo #3995 Show on time‑lapse

Morula with low number of cells

No cavity is visible.
Embryo is formed by very few large cells (up to 8).
Some lysed and/or vacuolized blastomeres are excluded from the embryo into the PVS (peri viteline space).

6
D
A
Very Low
Embryo #8544 Show on time‑lapse

Morula with degeneration/vacuolization of majority of the cells

Blastomeres undergo degeneration.
Degeneration is expressed by a total cell lysis or extreme vacuolization.
No further progress in development is achieved.

Arrested Embryo

Arrested/Degenerated

The embryos are arrested in their development.
There is no progress in cell cleavages.
Cells frequently undergo degeneration and vacuolization/lysis.
The development can stop at any time (from 2cc stage to more than 8–16cc).
No signs of the cell compaction are visible.

Unfertilized

Unfertilized/Uncleaved

No signs of pronuclei formation.
In some cases PN are present but no cleavage occurs.
The oocytes can be lysed soon after ICSI or can undergo degeneration during prolonged culture.
In case of extreme vacuole formation, the oocyte can increase its diameter and that is followed by the total lysis of the cell.

Implantation Chance

Implantation Chance by Grades

The implantation potential (IP) is an expression of the embryo's capability to achieve successful implantation. Because implantation is a multifactorial event, IP of embryos in our grading system is suggested based on the assumption that the embryos are ideally transferred into an ideal in-vivo environment.

Implantation potential
Grade 01
Hatching blastocyst
Grade 02
Expanded blastocyst
Grade 03
Expanding blastocyst
Grade 04
Blastocyst
Grade 05
Early blastocyst
Grade 06
Morula
Grade 07
Arrested Embryo
Grade 08
Unfertilised
Very High High Moderate Low Very Low No Chance
1AA Very High Embryo #7087
1AB Very High Embryo #8397
2AA Very High Embryo #10790
2AB Very High Embryo #4743
2AC Very High Embryo #15010
1BA High Embryo #8538
2BA High Embryo #10410
1BB High Embryo #4737
2BB High Embryo #3807
3AA High Embryo #10556
1BC Moderate Embryo #8613
2BC Moderate Embryo #5109
4AA Moderate Embryo #4000
3BA Moderate Embryo #7572
1CA Moderate Embryo #793
2CA Moderate Embryo #10409
3CA Moderate Embryo #4258
4BA Low Embryo #7574
5AA Low Embryo #4004
6AA Low Embryo #3576
4CA Low Embryo #10562
5CA Low Embryo #10555
6CA Very Low Embryo #3995
1DA Very Low Embryo #1455
2DA Very Low Embryo #10633
3DA Very Low Embryo #7362
4DA Very Low Embryo #6870
5DA Very Low Embryo #5838
6DA Very Low Embryo #8544
7D No Chance Embryo #3521
8D No Chance Embryo #5140
Very High High Moderate Low Very Low No Chance
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