At specific time points during embryo development the embryologist grades the appearance of embryos to determine the quality. There are multiple different grading systems but each system is designed to enable the embryologist to rank the embryos. The ranking is used to select the best quality embryo(s) for transfer to provide the highest chance of pregnancy. Once the embryo(s) have been selected for transfer the ranking of the remaining embryos is used to determine if there are surplus embryos of sufficient quality for freezing. The quality criteria for embryo freezing will differ between fertility centres so please talk to your clinic for specific details. Generally clinics only recommend freezing good quality embryos because these embryos are capable of surviving the freezing and thawing processes.
When the embryologist talks about embryo quality this is only based on the appearance of the embryo down the microscope and the grade assigned using a grading system. There are other factors that the embryologist cannot see which influence whether the embryo has the potential to implant such as the genetic status.
When fertilisation occurs (the union between an egg and sperm) the fertilised egg is a single cell which divides to form a 2 cell embryo. The process of dividing is referred to as cleavage. Over a course of days the cells of the embryo should undergo a series of divisions from 2 cells to 4 cells and so forth. The grading systems are designed for embryo at two specific stages of development:
- Days 2 & 3 of development after egg collection: Embryos at this stage are referred to as cleavage stage embryos.
- Days 5 onwards: Some embryos will have developed into blastocyst stage embryos.
Grading cleavage stage embryos (day 2 & 3 after egg collection)
There are 3 features that an embryologist will look at to determine the quality of a cleavage stage embryo.
- Number of cells: The number of cells is counted to make sure the embryo is not developing too slowly or quickly. Day 2 embryos ideally consist of 4 cells whereas day 3 embryos ideally consist of 6-8 cells
- Evenness of cell size: Ideally when a cell divides the two resulting cells should be the same size. If the division is uneven this means there is uneven distribution of the factors that cells need to develop normally. Embryos with a high degree of unevenness between the cells are assigned the lowest score because these embryos have a lower pregnancy rate.
- Amount of fragmentation: when a cell divides small pieces of cellular material can break off and form fragments. It is not unusual for an embryo to have a very small amount of fragments. The lowest grade is assigned to embryos with a high proportion of fragments.
Grading blastocyst stage embryos (day 5 onwards after egg collection)
A blastocyst is the point when the embryo becomes organised into two different cell components and a fluid cavity in the middle, usually occurring about 5 days after fertilization. There are 3 features that an embryologist will look at to determine the quality of a blastocyst stage embryo. Each feature is scored resulting in a code composed of a combination of 3 numbers and/or letters.
- Degree of expansion of the blastocyst: As the number of cells increases and the fluid filled cavity grows in size the degree of expansion of the blastocyst increases. Blastocysts that score highest will have expanded to the degree that it has started to hatch or might have fully hatched out from the “shell” which surrounds embryos called the zona pellucida. Blastocysts need to hatch out to allow implantation into the lining of the uterus.
- Appearance of the inner cell mass (ICM): The ICM is a cluster of cells which will hopefully develop into the baby. A tight cluster composed of many cells will score more highly than a very small cluster with very few cells. The lowest grade is assigned if there isn’t a cluster of cells visible down the microscope.
- Appearance of the trophectoderm: The trophectoderm is the outer layer of cells of a blastocyst that becomes the sac and placenta. The highest score is assigned if the trophectoderm is composed of a neat layer of many small cells with no gaps. Lower scores are assigned if there are large gaps and there are very few large cells forming the trophectoderm layer.
Please speak to your clinic for specific details on the grading system they use. It can be confusing because there are many different system and on a scale of 1 to 4 it could mean 1 is the best quality or is reversed and 4 is the best quality. Often clinics convert the grade into a quality category so that it is easier to talk about embryo quality. Rather than the combination of numbers or letters the embryologist may use the quality classifications of top, good, fair, poor and very poor. The embryologist will then be able to discuss up to date clinic success rates based on the quality of embryos available for transfer.
The title of the article is egg and embryo quality so you may be wondering when egg quality is going to be covered. I hope you can see from the article so far there are several features an embryologist can visualise at different stages of development to determine the quality of the embryo. There isn’t an equivalent established grading scheme for eggs to determine the quality and there are factors which determine egg quality that the embryologist cannot visualise down the microscope. The proportion of genetically abnormal eggs increases as women age so age is considered a predictor of egg quality. Genetically abnormal eggs can fertilise but most genetically abnormal embryos will not implant and result in a negative pregnancy test. A genetically abnormal embryo might implant but result in a miscarriage or a baby with a chromosomal disorder such as Down Syndrome.
Whilst there isn’t an established grading scheme for eggs there are features the embryologist looks for down the microscope. When the eggs are identified during egg collection they are surrounded by cells known as cumulus cells. The cloud of cumulus cells surrounding the egg prevents the embryologist from being able to visualise any features of the egg. The first opportunity the embryologist has to visualise the egg is after removal of these cells. The time the cumulus cells are removed is different depending upon the treatment type. For intracytoplasmic sperm injection (ICSI) the cumulus cells must be removed the same day as the egg collection to allow the embryologist to classify the maturity of the egg. There are two classifications for egg maturity:
- Mature: Mature eggs are referred to as Metaphase II (one of the phases of cell division) and at this stage are potentially capable of being fertilised. Not all mature eggs will fertilise and your clinic will be able to give you update to date data on fertilisation rates.
- Immature: Depending upon appearance immature eggs are classified as Metaphase I or Germinal Vesicle (GV) (other phases of cell division). GV is the most immature stage. It is not unusual for a small proportion of eggs to be immature. Immature eggs are not injected with sperm during ICSI.
For standard in vitro fertilisation (IVF) the cumulus cells are left surrounding the egg and a small amount of prepared sperm is added to the dish the same day as the egg collection before being cultured overnight. During fertilisation check the morning after egg collection the cumulus cells are removed to allow visualisation to determine if the egg has fertilised. It is only at this point that the embryologist will be able to see that an egg was immature and incapable of being fertilised. Once all the normally fertilised eggs have been identified they are cultured and graded at the specific time points as described above for embryos.
I hope this article has provided the basics on the quality checks performed on eggs and embryos. The grading of embryos is more comprehensive compared to eggs because there are many more features which can be visualised. The final recommendation on which embryo(s) to transfer and freeze is therefore based on the embryo rather than egg quality. There are differences between fertility centres so for more in depth information please talk to an embryologist at your clinic. They will be able to discuss about the grading system used to assess quality and latest success rates based on the embryo quality.