Embryo Grading and Oocyte Assessment

Embryo grading is a way to assess the quality of embryos in IVF. It involves checking factors like cell number, structure, and fragmentation under a microscope. Higher-grade embryos are generally more likely to result in a successful pregnancy, but grading is just one part of the process.

o   Day 0: Mature egg is inseminated

o   Day 1: Fertilization assessment

o   Day 2: Embryo is between 2-4 cells

o   Day 3: Embryo is between 6-8 cells

o   Day 4: Embryo is compacting (cell-to-cell adherence) or Morula (16-32 cells, cells adhered) or cavitation (CAV)

o   Day 5-7: Embryo shows early cavitation (CAV) or becomes Blastocyst

Egg Retrieval. After retrieving the eggs, they are placed in an incubator with carefully controlled conditions for several hours. An embryologist then removes the eggs from the incubator and checks if the eggs are mature.

There are 3 different stages of egg (oocyte) maturation:

• Germinal Vesicle (GV) = most immature oocyte
  • The egg has not yet begun meiosis
• Metaphase I (MI) = immature oocyte
  • The egg is in the first but not the second phase of meiosis (i.e. not fully mature)
• Metaphase II (MII) = mature oocyte
  • The egg is in the second phase of meiosis (i.e. mature) and ready for fertilization

Insemination. Mature eggs undergo Intracytoplasmic Sperm Injection (ICSI) and are placed back in the incubator. It is important that only mature eggs undergo ICSI since immature eggs may fertilize when injected with sperm but may not produce a viable ongoing pregnancy. Once an egg undergoes ICSI and is fertilized by sperm, it is called an embryo.

Fertilization Check and Grading. After insemination, the embryos are checked for fertilization but not graded. Embryos are graded (evaluated for development) at Day 3 and after Day 5 until they reach the blastocyst stage.

Blastocyst Stage. This stage occurs before the embryo implants in the uterus. Implanting at the blastocyst stage is better because it indicates the embryo has successfully overcome key developmental challenges. Blastocysts are evaluated by New Hope Fertility Center’s grading system.

Different embryo grading systems are used to help us determine which embryos to transfer. Embryo transfers occur either 3 days or 5 days after egg retrieval. Since embryos are developmentally different on these days, we use different grading systems for Day 3 embryos (called “Cleavage Stage”) and blastocysts (called “Blastocyst Stage”).

Cleavage Stage Embryo Grading (after fertilization but before blastocyst stage):

Embryos are graded on an A, B, C and D scale based on several metrics: the embryo’s rate of development on that particular day, fragmentation percentage, synchrony of cell division, and evenness of cell division.

Blastocyst Stage Embryo Grading (once embryo reached blastocyst stage):

Blastocysts are graded by the 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]

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 (good)
2. Several cells, loosely packed (fair)
3. Very few large cells (poor)

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

1. Many cells, tightly packed (good)
2. Few cells, forming a loose layer (fair)
3. Very few cells (poor)

It is important to note that embryo grading is subjective. An embryologist assesses an egg or embryo based on their own experience and expertise, but there is no guaranteed way to predict success. Embryos with lower grades can still lead to pregnancies while high-grade embryos may not always result in a successful outcome.