Embryo grading refers to the grading of embryos in the In Vitro Fertilization (IVF) culture, depending on their quality and number of cells present in the embryo. In treatments for assisted pregnancy like IVF, the resulting embryo has to be transferred to the uterus of the mother. As numerous embryos are formed during the procedure, it becomes necessary to select the best quality embryo for transferring in order to conceive successfully and bear a healthy child. Embryo grading is also used to select the embryos for freezing.
What Is Blastocyst?
An embryo on the fifth day of fertilization is referred to as a blastocyst. It has two different components: cell and a cavity filled with fluid. A good quality blastocyst must contain well defined Inner cell mass, blastocoel (fluid) cavity and the trophectoderm. As the embryo grows, these blastocoels enlarge, the shell thins and embryo hatches out. This is the ideal time for implantation.
Blastocyst grading is done as per the scoring system developed by Dr. David Gardner in late 1990’s. Though there are numerous other grading systems, this system is used widely due to its success rate. In this method of grading, three scores are given for three different qualities of the blastocyst embryo. They are: Blastocyst Development Stage, Inner Cell Mass quality and Trophectoderm quality.
Blastocyst Development Stage
In this stage of development, the size of blastocoel is determined for grading. The grades are assigned based on following parameters:
- Grade 1 - Blastocoels cavity is less than half of the volume of embryo.
- Grade 2 - Blastocoel is more than half of the embryo’s volume.
- Grade 3 - Blastocyst is full and blastocoel fills the embryo.
- Grade 4 - Expanded balstocyst with cavity bigger than embryo.
- Grade 5 - Hatching embryos.
- Grade 6 - Hatched embryos.
Inner Cell Mass (ICM) Quality
Inner cell mass is the cell mass which will develop into a fetus in developmental process of embryo. Here, Grade A is for ICM with many cells that are tightly packed. Grade B is for the ICM with several cells and loosely grouped ones. Grade C is for the ICM with very few cells.
Trophectoderm (TE) Quality
Trophectoderm, the outer covering which will form the placenta in the future, is also graded. Grade A is for the TE which has many cells making a cohesive layer. Grade B is for the TE with few cells making a loose epithelium, and Grade C is for the TE with very few but large sized cells.
The three components of the blastocysts are graded and the score is expressed in the order of Expansion Grade, ICM grade followed by the TE grade. An example score is 3BA, which means the balstocyst development score is 3 (full balstocyst), ICM score is B (several loosely grouped cells) and TE score is A (many cells forming a cohesive layer). The final score is used in deciding the quality of the embryo as a whole. A healthy and hatched embryo then is implanted in the uterus one to four days from the day of embryo transfer.