Embryo grading is complex process utilized by physicians and embryologists to help determine the best quality embryo based on morphology (appearance). Grading embryos begins at the time of the egg retrieval and continues throughout early embryonic development. A patient’s age, fertility history, and other information also help to determine the optimal day of transfer, the appropriate number of embryos to transfer, and exactly which embryos to transfer.
EMBRYO DEVELOPMENT TIMELINE
The Day of Egg Retrieval (Day 0) – During the in vitro fertilization process, 80 percent of eggs that are retrieved will be mature and only mature eggs will successfully fertilize. Eggs are fertilized through either:
- Microdroplet – In this process approximately one hundred thousand sperm are dropped onto the egg. The only way to know if the egg was mature was if the egg was actually fertilized; or
- CSI (intracytoplasmic sperm injection) – After choosing a normal sperm, the sperm is injected directly into the egg. After the egg is stripped of its supporting cells. The embryologist can then tell if the egg is mature and has the potential to fertilize.
Regardless of the fertilization method, the average fertilization rate with IVF is 70 to 75 percent.
Day 1 – After the embryologist determines which eggs have fertilized, the embryo is scored based on the number, position and size of the pronuclei. The appearance of two pronuclei is the first sign of successful fertilization as observed during IVF and is usually observed 18 hours after insemination. The two nuclei, one from the sperm and one from the egg should be equal in size and located in the center of the egg.
Day 2 – The embryos are scored based on the number of cells, number of nuclei and evenness of cell size. Ideally, the embryo should be made of 4 cells and each cell should only have one nucleus. Cells that have multiple nuclei are more likely to arise from an abnormal embryo. An embryo is graded based on how even in size the cells are. The more equal in size the higher the score.
Day 3 – The embryo is based on the number of cells and the evenness of cell size. This stage is sometimes referred to as the “cleavage stage” because the cells in the embryo are dividing but the embryo itself is not growing in size. The embryologist will look at the three dimensional distribution of cells, the degree of fragmentation and the embryos’ rate of development. Embryos that grow too quickly or too slowly are more likely to be abnormal. The embryo will be graded on how the cells in the embryos look. The system is presented in the following table:
Embryo Grade Description
Grade 4 Cells are of equal size; no fragmentation seen
Grade 3 Cells are of equal size; minor fragmentation only
Grade 2 Cells are of unequal size; moderate fragmentation to moderate fragmentation
Grade 1 Cells are of equal or unequal size; fragmentation is moderate to heavy
In Grade 1 and 2, the embryos appear to have the greatest potential for developing to the blastocyst stage. However, a grade 3 embryo may also be of good quality if its appearance can be explained by asynchronous cell division rather than by poor development.
Day 4 – Most normal embryos are at the morula stage on day 4. This looks like a big ball of cells rapidly dividing.
Day 5 – By the fifth day, embryos usually have undergone “compaction” or a fusion of cells to form the blastocoel. The blastocoel is a fluid filled structure that helps separate the trophectoderm and the inner cell mass. The best quality blastocyst has a good number of cells that are evenly sized and spaced. These cells become the placenta. The cells that make up the inner cell mass become the embryo.
There are two cell types in the day 5 embryo. The cells that form the Inner Cell Mass (ICM) will eventually grow into the fetus. The other cell type, the Trophectoderm Epithelium (TE), will grow into tissues needed during pregnancy (like the placenta). Together these cell types make a fluid filled sphere with the TE cells on the outside and the ICM inside. Both of these cell types are necessary to establish a healthy pregnancy. You cannot have a baby without a placenta and you cannot have a pregnancy without a fetus, so when we grade embryos at the blastocyst stage, we grade each of the cell types as well as to the fluid filled cavity or blastocoel.
There are many embryo grading systems that embryologists use to evaluate embryos for transfer. The ASRM (American Society of Reproductive Medicine) has attempted to establish a standard grading system, so when a patient or previous clinic is talking about “the quality of the embryos”, it can be interpreted and understood by the clinic she is now at, which is important in trying to understand why her cycle of IVF was unsuccessful and what “changes” can be made to result in a successful outcome. Even with standardized grading systems, which is still not universally used, evaluation and scoring of any stage of embryo development, from fertilization to a hatching blastocyst is critical to the experience and expertise of the embryologist.