Embryo Grading

During IVF, the embryos are cultured for up to six days and receive quality grades each day.

Day 0: Egg Retrieval and Insemination

Egg maturity is important because a mature egg has the best chance of being fertilized. There are three different stages of egg maturation:

  • Germinal vesicle (GV): The egg has not begun meiosis yet, so it is considered immature.
  • Metaphase I (MI): The egg is in the first phase of meiosis; however, it is still not completely mature because it has not entered the second phase of meiosis. This kind of immature egg may mature after a couple of hours of temperature-controlled incubation.
  • Metaphase II (MII): The egg is in the second phase of meiosis and is mature. Eggs at this stage of maturity are ready for fertilization.
    Egg quality is graded on a good/fair/poor scale

  • Good
    • Clear cytoplasm/normal shape
    • Single distinct polar body
    • Clear/thin zona pellucida
  • Fair
    • Slightly grainy cytoplasm/misshapen
    • Fragmented/abnormal polar body
    • Slightly pigmented/amorphous zona
    • Cytoplasmic bodies
    • PV debris
  • Poor
    • Dark/grainy cytoplasm/misshapen
    • Less than 1 polar body structure
    • Pigmented/thickened zona
    • Vacuoles
    • PV debris

Day One: Fertilization Check

Fertilization can be seen 16 to 22 hours post insemination. Normal fertilization is identified by exactly two pronuclei in the center of the single cell zygote. Fertilization is considered abnormal when there is only one pronucleus or when there are more than two pronuclei.

Day Two/Three: Multicell Grading

On day two the single cell zygote should divide into an embryo (approx. two to four cells). On day three the embryo should continue to divide (four to eight cells).

Embryo Quality:

  • Good: cells are symmetrical with clear cytoplasm
  • Fair: cells are slightly asymmetrical and/or have slight cytoplasmic irregularities
  • Poor: cells are significantly asymmetrical and/or have dark, grainy cytoplasm

Fragmentation: little bits of cytoplasm that escape during cellular division and stay within the embryo. The ranges of fragmentation are listed below from least to most heavy. Fragmentation ranging from A to B is most preferred.

  • A = No fragmentation
  • B = <10% fragmentation
  • C = 10–35% fragmentation
  • D = >35% fragmentation

Day Four

On day four, embryos begin their transition from a multicell embryo to a more advanced developmental stage. Embryos should begin compacting and forming morulae. Cells of a morula-stage embryo are not as distinct as in previous days; therefore, these embryos do not receive quality grades.

Day Five/Six: Blastocyst Stage

Each embryo that is at the blastocyst stage will be given a grade that consists of a number and 2 letters:

  • Blastocyst development stage - expansion and hatching status (1-6): Stages 1-2 are called “early blastocysts” at SDFC. We start grading at stage 3
  • Inner cell mass (ICM) score (1st letter grade)
  • Trophectoderm (TE) score (2nd letter grade)

At this stage, embryos should now have two distinctive cell layers: the inner cell mass (ICM) which becomes the fetus and the trophectoderm (TE), which are the outer layer of cells that will become the placenta. The number identifies how much the embryo has expanded and/or hatched and ranges from 1-6, with a 6 indicating that an embryo has completely hatched from its protective shell (zona pellucida). Stages 1-2 are called “early blastocysts” at SDFC. We start grading at stage 3. The next two letters then describe the ICM and TE, and are graded A, B or C. An A grade indicates that there are many cells that are tightly packed, B grade embryos have several cells that are loosely grouped, and C grade embryos have very few cells. Embryos that are late stage 3 and higher, and with A or B grades are biopsied and frozen on Day 5, 6, or 7. Early stage 3 embryos will remain in culture overnight to allow the embryo time to develop more cells, resulting in better survival of the biopsy procedure. Unfortunately, blastocysts with a C grade do not meet our criteria for biopsy and freezing.

Size and expansion Gardner’s Scale (SDFC’s former grading)
Early blastocyst: the blastocele is less than half the volume of the embryo (Early) SDFC will still use” Early”
Blastocyst: the blastocele is greater than or equal to half of the volume of the embryo. (Early) SDFC will still use” Early”
Full blastocyst: the blastocele completely fills the embryo. (Expanding Blast)
Expanded blastocyst: the blastocele volume is larger than that of the early embryo and the zona pellucida is thinning. (Expanded to Fully Expanded Blastocyst)
Hatching blastocyst: the trophectoderm has started to herniate through the zona pellucida. (Hatching Blastocyst)
Hatched blastocyst: the blastocyst has completely escaped from the zona pellucida. (Hatched Blastocyst)
ICM (Inner Cell mass)
tightly packed, many cells (Good)
loosely grouped, several cells (Fair)
very few cells (Poor)
Trophectoderm (TE)
many cells forming a tightly knit epithelium (Good)
few cells (Fair)
very few cells forming a loose epithelium (Poor)
Stage 1
Stage 1
Stage 2
Stage 2
Stage 3
Stage 3
Stage 4
Stage 4
Stage 5
Stage 5
Stage 6
Stage 6