In the process of In-Vitro Fertilization (IVF), embryo grading is performed, assessing the quality of the embryos– with Grade 1 generally being the best quality, and going down to Grade 4. Some centers do have different grading systems, but this is the most universally used.
Development of the Embryo
When the sperm has fertilized the egg cell, it forms the single cell zygote. This cell divides by the end of the first 24 hours, forming a 2-cell embryo. Usually the timing of the first inspection of the embryos will be in the morning of Day 2. At this stage, there should be a 4 cell-embryo, and by Day 3, it should be a 6 to 8-cell embryo.
The transfer of the embryo or embryos to the uterus usually takes place on Day 2 or Day 3. Sometimes, the decision is made wait until Day 5 for the transfer. At this stage, differentiation of cells has begun, and the embryo is now called a blastocyst. If a Grade 1 embryo is able to become a Grade 1 blastocyst before transfer, the chances of success (as measured by pregnancy rates) are much greater. However, there is also the risk that the embryo might not survive until Day 5.
Features of Grade 1 Embryos
The quality of embryos varies between individuals and within one individual, just as pregnancies vary in their nature. At the inspections, the quality of the embryos is graded, and this is done according to several criteria. In the Grade 1 embryo, the gradings will all be optimal. The criteria for grading is:
- The number of cells on a specific day. These will be the number anticipated, unlike in some embryos where cleaving of the cells may be a little slower. Embryos which cleave early appear to produce better results.
- The shape, size, smoothness of contour and consistency of the cells. In Grade 1, the cells will be absolutely regular, smooth and of equal size.
- The amount of fragmentation around the cells. These are fragments of cytoplasm which have come from the cells. In Grade 1, these will be minimal or totally absent.
- The nuclei of the cells will appear normal, as will the rest of the contents of the cells.
It was generally the practice to transfer several embryos to the uterus to maximize the possibility of at least one implanting and a pregnancy resulting. However, this brought about multiple pregnancies, with all the associated problems. If one is trying to have an Elective Single Embryo Transfer performed, with the aim being to achieve a singleton pregnancy, then a Grade 1 embryo is desirable.
Grade 1 embryos have the best chance of implanting successfully in the uterus and continuing on to a successful full term pregnancy. However, The grading system is no guarantee of success. It simply aids in making the best choices with the current knowledge available.