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Embryo Grading

Understanding Your Embryology Updates:

Key Concepts:

Day 0 = the day of your egg retrieval.
Day 2 and Day 4 = No updates on these days. Embryo is left in incubator, undisturbed.
Day 5 – 7 = embryo will reach blastocyst. Max up to day 7.
Morula (MOR) = is 2 stages before/behind the blastocyst stage.
Cavitating (CAV) = is 1 stage before/behind the blastocyst stage.

Oocyte = egg

Cultured = allowed to grow

Biopsy = take a small sample and send off to outside laboratory for testing

Blastocyst = final stage of IVF embryology development.

PGS testing/PGT-A = biopsy will be done once the embryo reaches the blastocyst stage.

Day 0 or Day 1: Assessing Egg Quality

  • MII – (metaphase 2 oocyte) A fully mature oocyte
  • MI- (metaphase 1 oocyte) A semi immature oocyte
  • GV- (Germinal Vesicle) A completely immature oocyte (less likely, but still has a CHANCE of fertilizing).
  • BZ- (Broken Zona/ Empty Zona) An oocyte in which the zona pellucida (the outer shell of the oocyte) has been compromised to the extent that it is no longer capable of being fertilized
  • DEG- (Degenerated) An oocyte or embryo that is no longer progressing

Day 0 or Day 1: Fertilization/Insemination

  • ICSI is when 1 sperm is injected into each mature oocyte to facilitate fertilization
  • Conventional insemination (cIVF) is when the oocytes are combined with the sperm and left to fertilize on their own
  • Day 1 ICSI is when ICSI is performed on Day 1, on oocytes that were previously immature on Day 0 but subsequently matured overnight

Insemination type:

ICSI is when 1 sperm is injected into each mature oocyte to facilitate fertilization

Conventional insemination is when the oocytes are combined with the sperm in a drop of media and left to fertilize on their own

Day 1 ICSI is when ICSI is performed on day 1 on oocytes that were previously immature on day 0 but subsequently matured overnight

Day 1-3: Monitoring Fertilized Egg Growth

  • 2PN- An oocyte with 2 pronuclei – this is considered normal fertilization
  • 1PN, 3PN, 4 PN etc. – These are oocytes with an abnormal number of pronuclei. We continue to culture (watched for changes in development).
  • PB – These are oocytes that failed to show signs of fertilization in the normal expected time frame (these can possibly still grow to blastocyst, but less likely).

Day 3-5: Cleavage embryo grading

Grade 1 – embryo with blastomeres of equal size, no cytoplasmic fragmentation
Grade 2 – embryo with blastomeres of equal size, minor cytoplasmic fragments or blebs
Grade 3 – embryo with blastomeres of unequal size, none or few cytoplasmic fragmentation
Grade 4 – embryo with blastomeres of unequal size, significant cytoplasmic fragmentation

Examples of Cleavage Grading in Embryology Report:

Example #1:

DAY 3

  • 1 Embryo, 6 cells grade 3

Interpretation: It has been 3 days since your egg retrieval. You have 1 embryo that has reached 6 cells (Image #1). The cleavage QUALITY so far is Grade 3 (Image #2). We will continue to monitor development.

Example #2:

DAY 3

  • 3 Embryos, 8 Cells Grade 2
  • 1 Embryo, >10 Cells
  • 1 Egg, BZ (Discarded)

Interpretation: It has been 3 days since your egg retrieval. You have 3 embryos that have reached 8 cells (Image #1). The cleavage QUALITY so far is Grade 2 (Image #2). You have 1 embryo that has more than 10 cells, which is now past the stage for cleavage grading. You have 1 egg that unfortunately did not fertilize, and has stopped progressing. We will continue to monitor the development of the other embryos.

Day 5-7: Blastocyst grading

Grade 1 – Early blastocyst; the blastocoel less than 50% of the volume of the embryo
Grade 2 – Blastocyst; the blastocoel more than 50% of the volume of the embryo
Grade 3 – Full blastocyst; the blastocoel completely fills the embryo
Grade 4 – Expanded blastocyst; the blastocoel volume is larger than that of the early embryo and the zona thinning
Grade 5 – Hatching blastocyst; the trophectoderm has started to herniate through the zona
Grade 6 – Fully Hatched blastocyst; the blastocyst has completely escaped from the zona

Inner Cell Mass (ICM) grading

A (Best) – Many cells, tightly packed
B – Several cells, loosely grouped
C – Few cells

Trophectoderm grading

A (Best) – Many cells in cohesive layer
B – Several cells in loose epithelium
C – Few cells

Blastocyst Grading correlates to MATURITY, not quality.

  • GRADE 1 indicated the EARLIEST stage
  • GRADE 6 is the LATEST stage.

The letters A, B, C correlate to QUALITY.

  • “B” indicates an average to ABOVE average quality embryo layer (Image #4).
  • “C” indicates an average to BELOW average embryo layer (Image #4).
  • Normal Quality: AA or BB.
  • Medium Quality: BC or CC.

Example: Blastocyst 4BC

  • “4” means Grade 4, which is an Expanded Blastocyst
  • “B” with an inner cell mass (this is the part of the embryo that becomes the fetus) that is made up of several cells that are loosely packed
  • “C” trophectoderm layer (this is the part of the embryo that becomes the placenta) that is comprised of few cells.

Examples of Blastocyst Grading in Embryology Report:

Example #1:

DAY 5

  • 2 Embryos, blastocyst (5BB, 5BC)
  • 1 Embryo, CAV

Interpretation:It has been 5 days since your egg retrieval. You have 2 embryos that have reached blastocysts (Image #5). The Blastocysts are Grade 5 (Image #5) Quality BB and BC (Image #4). We will biopsy and cryopreserve (freeze) the blastocysts today.

You have 1 embryo that is at the CAV stage (Image #1), which still has a reasonable chance at reaching blastocyst. This could be a sign that the egg came from a smaller follicle, therefore may develop slower. We will continue to monitor the development for this embryo.

Example #2:

DAY 6

  • 1 Blastocyst, 6BB
  • 3 Blastocysts (2 x 5BB, 1 x 5CC)
  • 1 Embryo, MOR (3PN)

Interpretation: It has been 6 days since your egg retrieval. You have 4 embryos that have reached blastocysts (Image #5). The Blastocysts are Grade 5 and 6 (Image #5) Quality BB and CC (Image #4). We will biopsy and cryopreserve (freeze) the blastocysts today.

You have 1 embryo that is at the MOR stage (Image #1), which still has a possible chance at reaching blastocyst. Although this could be a sign that the embryo may be just slower in developing, the 3PN suggests that this embryo is likely ABNORMAL (Image #1). We will continue to monitor development.

Understanding PGS / PGT-A Results: Genetic Testing is performed on BLASTOCYSTS. The embryo must be strong enough to survive the biopsy procedure.

  • Normal
  • Abnormal (you may further discuss this with the doctor).
    • Mosaic: This indicates that this abnormality was not found in ALL the biopsied cells.
    • High-Level Mosaic: This embryo has abnormalities in greater than 50% of the biopsied cells. Data indicates that “High-Level Mosaic” embryo will result in healthy, normal babies about 15% of the time since there are normal cells present which may possibly take over.
    • Low-Level Mosaic: The embryo has abnormalities in less than 50% of the cells biopsied. This embryo can still result in a normal pregnancy, but the rate is 20% lower than the PGS normal embryos.
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