In this second article on embryo development and selection we look at blastocyst grading at day 5. Blastocyst grading is similar to embryo grading which happens at day 2 and 3 which we discussed in our first article.
Both embryo grading and blastocyst grading are important stages in deciding which embryos have the best chance of a successful pregnancy.
Blastocyst grading on day 5
The structure of the blastocyst is very different to a day 2 or day 3 embryo. On day 5 the embryo that has been developed in the laboratory is called a blastocyst. At this stage the embryo has grown in the lab for 5 days and it has over 100-150 cells.
The embryo has reached the final stage before it is ready for implantation. It now has two groups of cells, the inner cell mass that will form the baby and trophectoderm cells that will make the placenta. The embryo is filled with a fluid filled cavity called blastocoel. Think of the blastocyst as a balloon with a small ball inside.
Who is eligible for blastocyst transfer?
Most of our patients have an embryo transfer at the blastocyst stage. Culture of embryos to the blastocyst stage will however require at least 3 good quality day 3 embryos.
This is because on day 3 your embryos might look identical and of good quality, making it hard to select one for your transfer. We therefore discuss with you the option to culture the embryos in the laboratory for 2 more days as at least one should grow to become a blastocyst. The chance for the embryos not to develop into blastocyst(s) is less than 8%.
What is blastocyst grading?
Similar with day 2 and 3 embryo grading, the blastocyst grading is based on morphological assessment ie. how the cells of the blastocyst look at a specific time. The only difference is that the blastocyst has so many cells and at this stage they cannot be counted. Alternatively, the embryologists, often with assistance from time lapse (Embryoscope) algorithms, assesses the development on the morning of day 5. The embryos should have now formed blastocysts with visible inner cell mass and trophectoderm cells.
So, lets see an example. The embryo in the figure above is a “4AA”. The first number ie. 4 in this case is always the number that grades the expansion on a scale from 1 to 6, with 1 being a less expanded blastocyst (early blastocyst) and 6 a fully expanded-hatched blastocyst. The higher expansion shows the blastocyst ability to burst from its shell to prepare for implantation.
The first letter following the number ie. the A corresponds to the grading of the appearance of inner cell mass. The second letter A corresponds to the appearance of the trophectoderm cells. These cell types are graded from A to C, with A being the highest grade (many cells) and C the lowest (few cells).
Therefore, the blastocyst in the photo above is an 4AA, meaning a fully expanded blastocyst with many inner cell mass cells and many trophectoderm cells.
What if some of my embryos are not top-quality blastocysts?
Even if all embryos look great on day 2 or 3 not all of these will develop into blastocysts and this is a normal phenomenon. Not all embryos are healthy and robust hence they fail to grow into a blastocyst. Think of it as self-selection.
Blastocyst development varies from patient to patient. Around 30% of our patients will have at least a second blastocyst of good quality that could be cryopreserved. If however a blastocyst is not grade A or B and has been graded as C we will not be able to cryopreserve it because the embryo has very low chances to survive the freezing and warming process and achieve a pregnancy. Assessing the embryo morphology is an important factor but we have seen for years and it well documented in literature that embryos selected for fresh transfer graded as CC or BC can lead to successful pregnancies. So don’t panic or feel disheartened as these embryos still have chances for a live birth.