Blastocyst Stage Embryo Culture

What Is A Blastocyst?

During conception inside a woman's body, the egg is fertilized by sperm in the fallopian tube and an embryo is formed. Fertilization usually happens in the part of the fallopian tube that is closest to the ovary and the embryo continues to develop (through cell division) as it travels back down the tube towards the uterus. The embryo usually reaches the uterine cavity about 5 or 6 days after fertilization. At this time, it is a blastocyst, or an embryo made up of hundreds of cells that started as a single cell created from a fertilized egg.  

In an IVF cycle, a blastocyst forms in a culture system in a laboratory.  Eggs are retrieved from a woman's ovaries, fertilized with sperm, and an embryo is created.  The embryo divides and multiplies its cells over 5 to 6 days to become a blastocyst.   Embryos that survive to this stage of development have a high implantation potential once transferred into the uterine cavity.

Embryo Transfer

Allowing the embryos to grow from Day 3 (cleavage stage) to Day 5/6 (blastocyst) allows selection of better embryos for pregnancy. The primary benefit to the patient of using blastocyst stage embryo culture is that fewer embryos need be replaced into the patient because each day 5 embryo has a higher chance of implanting than a day 3 embryo does. Transferring fewer embryos greatly reduces the risk for twin or triplet pregnancies and the associated mom/baby risks.  Additionally, embryos that continue to grow to the blastocyst stage have a lower rate of chromosomal errors and therefore a higher chance of resulting in a healthy baby.

A Patient's Guide to Embryo Grading

Grade 1

The embryo has the appropriate cell number for stage of development (eg., on day three, embryos should have 8 cells, also known as blastomeres) and the cells are even in their size and shape. The embryos should have little or no fragmentation (small amounts of extracellular debris) between cells. 

Grade 2

The embryo has the correct development of cell numbers (6-8 cells on day three), with a small amount of fragmentation (<20% fragmentation), or slight uneveness of blastomere sizes.

Grade 3

The embryo contains blastomeres that are uneven in size and has more than 30% fragmentation.  Embryos developing slowly will also receive a grade of 2 or 3 even if they are normal otherwise.

Grade 4

The embryo contains very few, if any, viable appearing cells and has a high percent fragmentation (over 50%). 

Blastocysts are graded in the following manner:


Blastocysts range from 1 to 6 in terms of expansion (or how large they are).  Early blastocysts, with fewer cells, are graded as 1 or 2.  Because these have fewer cells and a poorly defined inner cell mass and trophectoderm (see below), they are not given any letter grades like the more advanced blastocysts are.  Blastocysts that are further along in development are given expansion grades of 3 through 6, where 6 indicates an embryo that has completely hatched out of the zona pellucida (shell).

Inner Cell Mass

The inner cell mass represents the collection of cells that are destined to differentiate into the fetus/baby.  This group of cells is given a letter grade of either A, B, C, or D.  Grades A and B are most desirable and reflect a good number of cells with normal appearances. 


The trophectoderm represents the collection of cells that will become the placenta and are responsible for the initial invasion and implantation of the embryo into the uterine lining.  This is also given letter grades of A, B, C or D.  Just as with the inner cell mass, grades A and B are most desirable and reperesent an excellent number and appearance of  the cells.

Though embryos are graded to signify their stage of development, it is important to remember that the grade does not always reflect the true potential of an embryo.  Specifically, there are embryos that receive less than ideal grades that frequently create beautiful babies. Embryo grading helps us detemine how many embryos to transfer.  It may also be diagnostic by showing us the appearance of embryos and clarifying whether there is concern regarding the quality of  eggs or sperm.  Finally it is important to remember that embryo grading is subjective, and that IVF programs may vary somewhat in assigning grades to embryos.

Embryo Images

Embryo Grading Embryo Grading Embryo Grading Embryo Grading

   12 Cell Grade 1     2 Cell Grade 1        2 Cell Grade 2        4 Cell Grade 1

Embryo Grading  Embryo Grading Embryo Grading Embryo Grading

   4 Cell Grade 1          4 Cell Grade 2        5 Cell Grade 1       5 Cell Grade 2

Embryo Grading Embryo Grading Embryo Grading Embryo Grading

   6 Cell Grade 3      8 Cell Grade 1         8 Cell Grade 1       8 Cell Grade 2

Embryo Grading Embryo Grading Embryo Grading Embryo Grading

   8 Cell Grade 3         Blastocyst             Blastocyst          Hatched Blastocyst

                              Grade A  (Day 5)    Grade A (Day 6)