A blastocyst is an embryo which has developed to the point of having 2 different cell components and a fluid cavity. Human embryos, in culture in an IVF lab, or developing naturally in the female body, usually reach the blastocyst stage by day 5 after fertilization. Blastocyst transfer with IVF can give high pregnancy success rates with very low risk of triplets.

Picture of blastocyst at time of embryo transfer
Blastocyst picture of a high quality human blastocyst embryo 5 days after in vitro fertilization
The clump of cells in the 10 to 12 o'clock area in the picture make up the inner cell mass which become the fetus

Day 5 human blastocyst picture showing inner cell mass
Another image of a high quality human blastocyst
The cells which will become the fetus are in the area marked as "ICM" (inner cell mass)
The blastocoel cavity in the center is marked as "C"
The trophectoderm cells that will form the placenta surround the cavity - one is marked with a "T"

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Blastocyst Development and Blastocyst Quality Grading (or scoring)

There are three distinct parts to a blastocyst - the two cell types and the fluid cavity. As development of blastocysts progresses, the cells in each of the 2 components divide and the fluid cavity enlarges. With continued blastocyst development the embryo expands, the shell thins, and it hatches out of its shell. Blastocyst implantation into the lining of the uterus can then begin.

Our IVF clinic and many others that do blastocyst culture and transfer use a blastocyst scoring system originally developed and published by Dr. David Gardner in the late 1990's. Dr. Gardner is well known for being first to develop a commercially available embryo culture media system that supported high rates of viable blastocyst development. There are other grading systems for blastocysts , but this one is the most widely used.

This blastocyst grading system assigns 3 separate quality scores to each blastocyst embryo:

Blastocyst development stage - expansion and hatching status
Inner cell mass (ICM) score, or quality
Trophectoderm (TE) score, or quality

Expansion grade	Blastocyst development and stage status
1	Blastocoel cavity less than half the volume of the embryo
2	Blastocoel cavity more than half the volume of the embryo
3	Full blastocyst, cavity completely filling the embryo
4	Expanded blastocyst, cavity larger than the embryo, with thinning of the shell
5	Hatching out of the shell
6	Hatched out of the shell

ICM grade	Inner cell mass quality
A	Many cells, tightly packed
B	Several cells, loosely grouped
C	Very few cells

TE grade	Trophectoderm quality
A	Many cells, forming a cohesive layer
B	Few cells, forming a loose epithelium
C	Very few large cells

Blastocysts are given a quality grade for each of the 3 components and the score is expressed with the expansion grade listed first, the inner cell mass grade listed second and the trophectoderm grade third. For example, a blastocyst quality grade of 4AB means that the blastocyst is expanded (grade 4), has many tightly packed cells in the inner cell mass (grade A), and has a trophectoderm with few cells forming a loose epithelium (grade B). See the examples of blastocyst grading pictures below.

It should also be mentioned that other blastocyst grading systems sometimes use more letter grades than the Gardner system. For example some clinics will give a grade D to a poor quality blastocyst. Other systems give component cells grades of D or even E in addition to the A, B and C grades described above.

We have even seen records from some IVF labs that give blastocysts a single letter grade, and no number at all. For example the blasts are simply graded as A's or B's or C's, etc. This kind of system is easier for the lab personnel to deal with, but at the cost of being less descriptive. There would also be some loss of ability to correlate grading of the blastocysts with expectations for a successful pregnancy outcome.

Early blastocyst grading photos:


Early blastocyst	Early blastocyst	1AB
Cavity very small = Can't yet grade Inner cell mass = Can't yet grade Trophectoderm = Can't yet grade	Cavity very small = Can't yet grade Inner cell mass = Can't yet grade Trophectoderm = Can't yet grade	Cavity < 1/2 embryo's volume = 1 Inner cell mass = A Trophectoderm = B

Intermediate blastocyst grading pictures:


2BB	2BC	3BB
Cavity > 1/2 embryo's volume = 2 Inner cell mass = B Trophectoderm = B	Cavity > 1/2 embryo's volume = 2 Inner cell mass = B Trophectoderm = C	Cavity completely fills embryo = 3 Inner cell mass = B Trophectoderm = B

Large, well expanded blastocyst grading images:


4AA	4AB	4BB
Well expanded = 4 Inner cell mass = A Trophectoderm = A	Well expanded = 4 Inner cell mass = A Trophectoderm = B	Well expanded = 4 Inner cell mass = B Trophectoderm = B

Hatching and hatched blastocyst grading pictures:

5AA

Blastocyst hatching out of shell = 5
Inner cell mass = A
Trophectoderm = A

Picture of a hatching blastocyst on day 6 of in vitro culture

It is almost completely hatched out of its shell (seen at lower left of image)
After a blastocyst hatches, it begins to invade into the uterine lining to "implant"
This embryo has a quality grade of 5AA (inner cell mass seen at the bottom is out of the plane of focus)
The dark areas at the top and lower right of the photo are shadows in the drop of culture media

Completely hatched human blastocyst picture

6AA

Blastocyst hatching out of shell = 6
Inner cell mass = A
Trophectoderm = A

Completely hatched blastocyst

This embryo has a quality grade of 6AA
The tightly packed inner cell mass seen at 3 to 4 o'clock in this picture

A healthy blastocyst will implant within about one to four days following a day 5 transfer, with invasion of the cells into the uterine lining occurring soon after blastocyst hatching.

More pictures of blastocysts

Pictures of human eggs and embryos at other stages of development

Babies who had our help

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