Understanding the Embryo

Embryos play a crucial role in the realm of human development. Here’s a detailed guide to navigate through their complexities.


An embryo originates from a fertilized egg. When discussing embryos, two primary aspects come into focus: their stage and grade.



  • This is day 1 after fertilization (ie “day 1 embryo”)
  • Embryos aren’t graded at this juncture.
  • The presence of two pronuclei indicates successful fertilization.


  • This is called “cleavage stage,” which is assessed on day 3 after fertilization (ie “day 3 embryo”)
  • At this point the embryos are graded on the basis of how many cells it has and how much fragmentation.
  • Fragmentation has to do with amount of breakage/debris around the embryo.
    • Fragmentation is graded 1 to 4
      • Grade 1 means low fragmentation 
      • Grade 4 means high fragmentation
    • Less fragmentation is associated with better implantation
  • The number of cells is then graded.
    • A “good” grading for a day 3  embryo is 8-12 cells grade 1 
    • A “bad” grading for a day 3 embryos is anything less than 8 cells and grade 3 fragmentation (ie; 4 cells grade 3 )


  • The last stage of an embryo is called the blastocyst stage”
  • This occurs 5-7 days after fertilization 

In summary, embryos are classified into day 1, day 3, or day 5 stages, signifying the developmental STAGE of the embryo.


Approximately 50% of embryos reach the blastocyst stage 5 days after fertilization. 

Sometimes embryos achieve blastocyst stage after 6 or even 7 days after fertilization 

A typical blastocyst has:

  1. inner cell mass
  2. trophectoderm (cells that become placenta)
  3. blastocele cavity


A blastocyst is graded based on its size, the appearance of the inner cell mass and trophectoderm

For example, a good quality embryo is graded 4AB

The first number refers to the size of the embryo

  •  1-2 are very small early blastocysts where an expanded blastocele cavity cannot be easily seen
  • 3 is a fully expanded blastocyst
  • 4 is a blastocyst with a larger blastocele
  • 5 means it has started to hatch (come out of the shell) 
  • 6 means it has completely come out of its shell.

The first letter refers to the inner cell mass. 

  • The inner cell mass are the cells that will eventually become a baby.
  • They are graded A-C
    • A being best
    • B being second best 
    • C being last

 The second letter grades the cells around the embryo called the trophoblasts

  • These are cells that will eventually become placenta
  • When an embryos is biopsied for PGT, these are the cells that get biopsied
  • They are graded A-C
    • A being best
    • B being second best 
    • C being last

Let’s look at some examples:

5AA is a top quality blastocyst

  • 5 means it has started to hatch
  • The first “A” refers to the grade of the inner cell mass (remember, A is best!)
  • The second “A”  refers to the grade of the trophoblast layer 

3BC is a low quality blastocyst 

  • 3 means it is smaller ( 3)
  • The first “B” means it has a lower grade inner cell mass 
  • The second letter “C” means it has an even lower grade trophoblast layer ( C) 

Most blastocysts are in the range of : 3AB0, 4BB0, 5BB0

Another aspect to remember about your blastocyst is whether they reached the blastocyst stage on day 5, day 6 or day 7 of development

  • Day 5 blastocyst have a slightly better implantation rate than day 6 blastocysts and subsequently day 6 blastocysts have a lower implantation rate than day 7 blastocysts.


Day 5 embryos (or blastocysts) are graded based on size, appearance of inner cell mass, appearance of trophoblast.

This is called morphology.

All of it is simply a way to describe a book by its cover. None of these descriptions really tell us anything about the genetics of an embryo

The number of chromosomes as well and location or arrangement of chromosomes is a KEY factor in determining the implantation potential of an embryo.

An embryo with perfect morphology can have abnormal genetics.
An embryo with poor morphology can have normal genetics. 

While morphology does play a factor, the genetics of an embryo is currently the most important predictor of implantation.

Markham Fertility Centre offers genetic counselling as part of our approach to personalized and informed reproductive care. Reach our genetic counselling team here.