Embryo Development
Once your eggs have been collected, the embryology team will become your point of contact for finding out how things are progressing. The information here aims to help support your understanding of this process, and it can be a lot of new information to take in at once. Depending on how long your embryos have been in the lab, the update will be different.
The day after your egg collection, you can expect to hear about how many of your eggs have successfully fertilised. We call a successfully fertilised egg a “2PN”. This is because the genetic material from the egg and the sperm can be identified as two small circles or pronuclei (Figure 1). At this stage, it can be difficult to assess the quality of an egg, so in general, we wait until it starts dividing.
The next call from the embryology team is normally day 3. At this point, the team will let you know how many fertilised eggs have divided, what stage they have reached and their grade. If you go on to day 5, the team will let you know how many embryos have reached the blastocyst stage and are suitable for transfer or freezing, depending on your treatment plan. Any embryo not transferred or frozen on day 5 that is still developing will be graded again on day 6. You will get a phone call letting you know the grades on day 6 if your day 5 information did not have a final grade for all your embryos.
Day 0 (day of egg collection): No embryo grade is assigned to eggs on this day. Oocyte quality can be graded for ICSI or egg freezing cases only.
Day 1: The day after the egg collection, fertilisation is expected to be visible. No embryo grade is assigned at this stage, as fertilised eggs have not started cell division.
Day 2: Early cell division of fertilised oocytes should be observed at this stage. Grading will be based on the pattern of cell division, cell symmetry and whether any cell fragmentation is present.
Day 3: Further cells will be observed at this stage. Grading will be based on the continued development of the embryo, the pattern of cell division, cell symmetry and whether any cell fragmentation is present.
Typically, a higher number of cells, symmetrical division and lower / no fragmentation is preferable.
Day 4: As embryos accelerate their cell division, they will enter a stage of cellular compaction (forming a tight structure of cells). An embryo is called a Morula at this stage. No specific grade is assigned to a Morula.
Day 5: Some embryos will advance to form a Blastocyst. The embryo at this stage is complex, and the grading is detailed as a result.
A Blastocyst is graded on:
- How much growth the blastocyst has undergone, and how far it has expanded.
- The cluster of cells inside the blastocyst are called the Inner Cell Mass (ICM). These cells which will give rise to the developing foetus.
- Peripheral cells surrounding the blastocyst are called the Trophectoderm Cells (TE). These cells will give rise to the placenta.
Day 6: Some blastocysts can be reevaluated on day 6, giving the embryo additional time to grow and improve its development. The same grading scale will be assigned to blastocysts on day 6: expansion rate, inner cell mass grade, and trophectoderm cell grade.
Understanding Embryo Grading
The grade given to an embryo is a combination of their appearance and pattern of their development as observed by the embryologist. Embryo observations are performed under a microscope or using time-lapse technology.
The developmental stage between fertilisation and day 3 is referred to as cleavage stage. Between day 3 and day 5, the embryos should develop into blastocysts. As you can see in the images below, embryos change a lot between these stages, and so different grading schemes are used to describe each one (Figure 1).
The number relates to the physical number of cells within the embryo.
The grade is associated with the quality of the cells, which includes cell division patterns, cell symmetry and the presence of fragmentation.
Grade 1 Embryo of 6-8 cells (Day 3), even symmetrical divisions, no or <10% fragmentation
Grade 2 Embryo of 6 -10 cells, (Day 3), uneven or irregular cells, asymmetrical divisions, 10-25% fragmentation
Grade 3 Embryo of slower (or faster) development, uneven cells, asymmetrical divisions, >50% fragmentation
Grade 4 Embryo that has stopped developing and is not suitable for use.
Blastocyst Stage Embryos (Gardner Grading System); D5 information
The number is associated with the rate of expansion or how big the blastocyst has grown. This scale is 1-6, 1 being the least expanded and 6 being the most expanded.
The first letter is associated with the appearance of the inner cell mass. This scale is A, B or C. ‘C’ being the weakest and ‘A’ being the strongest.
The second letter is associated with the appearance of the trophectoderm cells. This scale is also A, B or C. ‘C’ being the weakest and ‘A’ being the strongest.
