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Blastocyst Stage Embryo

Atlanta IVF Treatments

During conception, 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 about a hundred cells.

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 us to select the embryos with the highest chance of success. The primary benefit to the patient of using blastocyst stage embryo culture is that fewer embryos need be transferred 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 risks to the mother and child. 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.

Grading Blastocysts


Blastocysts sizes are measured on a scale of 1 to 6. 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, or C. Grades A and B are most desirable and reflect a good number of cells with normal appearances. Therefore only those cells graded A or B are selected to be cryopreserved.


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, or C. Just as with the inner cell mass, grades A and B are most desirable.

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 determine how many embryos to transfer. It is important to remember that embryo grading is subjective and that IVF programs may vary somewhat in assigning grades to embryos.

Embryo Images

2 Cell Grade 1 4 Cell Grade 1 8 Cell Grade 1 8 Cell Grade 2 8 Cell Grade 3 Blastocyst

2 Cell Grade (1) 4 Cell Grade (1) 8 Cell Grade (1) 8 Cell Grade (2) 8 Cell Grade (3) Blastocyst

Hatched Blastocyst

Hatched Blastocyst

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