Oocyte assessment:

  • GV = Germinal vesicle (most immature)
  • MI = Metaphase I oocyte (immature)
  • MII = Metaphase II oocyte (mature)

Typical developmental timeline for human embryos:

  • Day 0: Mature egg is inseminated
  • Day 1: Fertilization assessment
  • Day 2: Embryo is between 2-4 cells
  • Day 3: Embryo is between 6-8 cells
  • Day 4: Embryo is compacting ( cell-to-cell adherence) or Morula (16-32 cells, cells adhered ) or cavitation (CAV)
  • Day 5-7: Embryo shows early cavitation (CAV) or becomes Blastocyst

How to grade a cleavage stage embryo:  After fertilization, but before blastocyst stage- Embryos are graded on an A, B, C and D scale which is based upon the rate of development on that particular day, fragmentation percentage, synchrony of cell division, and evenness of cell division.cleavage-grading-sys

Once the embryo has reached blastocyst stage- Blastocysts are graded by total number of cells, cohesiveness of the cells making up the ICM (inner cell mass) and trophectoderm, and the expansion of the blastocoel (inner fluid filled area of a blastocyst). The ICM becomes the baby and the trophectoderm becomes the sac and placenta.

A) The first number corresponds to the progression of the blastocyst (1-6). A higher number corresponds to a more advanced stage in blastocyst development. Blastocyst

  1. Early blastocyst (blastocoel cavity < half the volume of the embryo) (EBL)
  2. Blastocyst (blastocoel cavity ≥ half the volume of the embryo) (BL)
  3. Blastocyst (blastocoel cavity completely fills the embryo) (BL)
  4. Expanded blastocyst (blastocoel cavity > original volume of the embryo, zona is thinned) (EXBL)
  5. Hatching blastocyst (trophectoderm has started to protrude through the zona )(HBL)
  6. Completely hatched blastocyst (blastocyst has completely escaped from the zona ) (CHBL)

B) The second number corresponds to the quality of the ICM (inner cell mass) (1= best quality)

  1. Many cells, forming a cohesive layer
  2. Few cells, forming a loose layer
  3. Very few large cells

C) The third number corresponds to the quality of the trophectoderm (1= best quality)

  1. Many cells, tightly packed
  2. Several cells, loosely packed
  3. Very few cells