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Assisted Fertility Program Embryologist, Dr. Charlie Lu

Many people undergoing IVF have questions about how the embryos are graded, prior to their embryo transfer. Embryos are graded in our CAP accredited embryology lab by our experienced embryologist.

Each day they are assessed by our embryologist to monitor their growth and development. Research favors inserting embryos once they reach a blastocyst. Culturing the embryos in the lab for 5 days or until they reach a blastocyst is recommended if the embryos are developing well. It is important to know the stages of embryo development in order to better understand how embryos are graded.

Embryo Development, Days 1-5

On the day of the egg retrieval, the woman’s eggs are fertilized with her partner’s or donor sperm. The eggs may be mixed with the sperm and allowed to fertilize naturally or we may use Intracytoplasmic sperm injection (ICSI) . ICSI involves injecting a healthy looking sperm directly into the egg to increase chances of fertilization for cases of male infertility or poor egg quality. The first step in the process of grading embryos is to determine if fertilization has actually occurred

.embryo grading

Day 1 Embryos: The first day following the egg retrieval, the embryos are assessed to determine if fertilization has occurred. This is commonly known as the 2pn stage. When looking under the microscope, there will be two pro-nuclei which appear in the center of the egg indicating that DNA from both parents is present and fertilization has occurred.

Day 2 embryos: The embryos will split or divide and two distinct cells will appear. The embryos then begin to divide and multiply each day.

Day 3 embryos: on Day 3, the embryos continue dividing and you can start to see more cells begin form.  This is commonly referred to as the cleavage stage. The three main things we look for on day three of embryo development is the number of cells, the symmetry of the cells and if there is any fragmentation present.

Symmetrical cells where the cells are all relatively the same shape or size is preferred, however some embryos may have asymmetric cells. Fragmentation can be described as embryo fragments within or surrounding the embryo which have broken off during the division process. Little to no fragmentation is preferred for the best implantation rates to occur. However, normal pregnancies can result even if the embryo has some fragmentation or asymmetry present.

On day three, a symmetrical 8 cell embryo with little to no fragmentation is preferable however not all embryos will have 8 cells on day three; many will have between 6-10 cells.  Embryos with 4 cells of fewer on day three, have a lower chance of implanting. Depending on the quality of the embryos and if they are growing well, your physician may opt to perform your embryo transfer either on day 3 or 5.

Day 4 embryos: On day four, the embryos continue to divide rapidly and are monitored in the lab. Day 4 is also known as the morula stage where anywhere from 16-30 cells may be present.  Typically, if the cells have reached a morula, they will likely make it to the blastocyst stage(around 120 cells) the next day.

Day 5 embryos: By the 5th day, the embryos should have reached a blastocyst and contain around 120 cells. At this point, the blastocyst is ready to hatch out of its outer shell which is commonly known as a hatching blastocyst and implant within the uterus. Many times, before the embryo transfer , the blastocyst may be hatching or already hatched. If hatching has not occured, assisted hatching is a manual procedure sometimes used with IVF which involves drilling several small holes in the outer shell of the embryo. This is thought to help the embryo expend less energy hatching out of the shell so it can spend more energy on implanting in the uterus.

It is important to note that some embryos grow at a different rate than others and may take necessarily reach a blastocyst on day 5 but instead day 6.

Day 3 embryo grading

Grading an embryo on day three includes an assessment of the amount of fragmentation present, the amount of cells present and if there is cell symmetry. Grading may vary based on the IVF lab used. Some labs will grade embryos by the number of cells (4, 5, 6, 8, 10, etc) followed by a letter grade such as A, B or C which describes the quality. For example; 8-A is an 8 cell embryo with an A grade.  However, some labs may list the number of cells followed by a number grade such as 1, 2 or 3. For example; 8-1 an 8 cell embryo with a 1 grade.

A grade embryos (Grade 1) Indicates a good quality embryo in which the cells are symmetrical and little to no fragmentation present. An embryo 8 cell embryo graded a 1, will have a very good chance of implanting.

B grade embryos (Grade 2) Indicates a fair quality embryo which may have some asymmetry or fragmentation present but still has a good to fair chance of implanting.

C grade embryos (Grade 3) indicates a poor quality embryo with moderate fragmentation, asymmetrical cells or low number of cells. Grade 3 embryos will have a lower chance of implanting in the uterus, however many grade C embryos do go on to establish normal healthy pregnancies.

Grade D embryos (Grade 4) Indicates a very poor quality embryo with severe fragmentation and asymmetrical cells. These embryos may be slow growing or contain a very low number of cells. It is unlikely, this embryo will result in a successful pregnancy however it is still possible.

Day 5 embryo grading:

Once the embryos reach a blastocyst, they are graded a little differently. Day 5 embryos are graded based on the appearance of the inner cell mass and by the appearance of the trophectoderm (outer cell mass). They are both assigned a grade which includes; A-D.

An ideal embryo has both the inner cell mass and trophectoderm graded as an A, which would be an AA embryo. An AA graded Embryo is the highest graded and will have a good chance at implanting.

Inner cell mass grading(mass in the middle that forms the baby)

  1. (Good) Numerous cells in the epithelium, cells appear healthy and are tightly packed.
  1. (Fair) Large cells which are loosely packed.
  1. (Poor) Few Cells, inner cell mass not distinguishable.
  1. D- Degenerative cells

Trophectoderm Grading (outer cell mass)(cells around the outer edge that form the placenta,or afterbirth)

  1. (Good) Many Cells forming an epithelium.
  1. (Fair) A few large cells forming an epithelium.
  1. (Poor) Few cells, that appear poor or uneven.
  1. Degenerative appearance.