By Galia Tsangkalova – Embryologist, Lab Director
The most commonly used blastocyst grading system is the one proposed by Gardner and Schoolcraft. Every blastocyst is graded by using one number and two letters.
The number indicates the expansion level which results from the comparison of the blastocoel cavity to the volume of the embryo.
The first letter indicates the quality of the ICM while the second letter indicates the quality of the TE, both of which are evaluated by the number and the cohesiveness of the constituent cells.
|1 Early blastocyst(Blastocoel cavity less than half the volume of the embryo)||A Tightly packed and many cells||A Many cells forming a cohesive epithelium|
|2 Blastocyst(Blastocoel cavity greater than or equal to half of the volume of the embryo)||B Loosely grouped and several cells||B Few cells forming a loose epithelium|
|3 Full blastocyst(Blastocoel completely fills the embryo)||C Very few cells||C Very few cells struggling to form a very loose epithelium|
|4 Expanded blastocyst(Blastocel volume is larger than that of the early embryo and the zona pellucida is thinning)|
|5 Hatching blastocyst(TE has started to herniate through the zona pellucida)|
|6 Hatched blastocyst(Blastocyst has completely escaped from the zona pellucida)|
ICM and TE Quality
The black arrows point the ICM whereas the white arrows point the TE of the blastocyst.
The above is a 4AB blastocyst meaning that the blastocyst is an expanded one (large blastocoel volume and thin ZP), having an excellent ICM of A quality (big with many tightly packed cells) and a promising but not excellent TE of B quality (few cells forming a relatively loose epithelium).