## 4 Morphology of the Blastocyst and Quality Assessment
### Blastocyst Visual Assessment  
Generally, the degree of blastocyst expansion in embryo development stage can be characterized into several categories, and this depends on the grading scheme. For example, the numerical scoring system presented by the Istanbul consensus [20], shows many studies have assessed the blastocyst stage according to the morphological appearance of blastocolel cavity, ZP thickness, ICM, and TE cells [35]. However, [PERSON] et al. [37] (who created [PERSON]'s grading system) have suggested that the grading of blastocysts mainly focuses on three morphological appearances of the embryo, i.e., the rate of embryo size expansion, the analysis of ICM, and the analysis of TE cells. In assessing the fertilized embryo, the degree of blastocyst expansion presents the capability of the cultured embryo to progressively enlarge the volume up to the hatching process [35]. Meanwhile, the ICM cell will turn to be a zygote or fetus [16; 26; 35] and therefore it is crucial to carefully grading the quality of ICM cell throughout the IVF selection procedure. In addition, the TE score is also one of the most important parameters to assess concerning embryo selection especially among older women (\(\geq\)35 years of age) [30] to ensure high successful rate in live birth.  
Figure 3: Blastocyst formation at Day 5-4 AA (hatching) and development into ICM, TE, ZP and blastocolel.  
The [PERSON]'s grading system [38; 39] evaluated blastocysts based on their morphology, i.e., the shape, size, and appearance, and has been used as a reference by many IVF laboratories as a tool to help embryologists measuring blastocyst growth and the quality of developed embryo. The [PERSON] grading system sequentially assigns three different quality scores of each embryo based on blastocyst development stage (expansion and hatching status), ICM score, and TE score, or quality. Based on these three components (degree of expansion, ICM, and TE), blastocysts are given a quality grade of each, and the score is expressed with the expansion grade listed first, the ICM grade listed second and the TE grade third. As an example, the grading of blastocyst quality based on [PERSON] scoring system has widely been used by many researchers and practitioners as presented in Figure 4. The embryo scoring, being performed mainly via visual evaluation based on embryologist experience, may often result in different interpretations of embryo quality [34]. The ability of embryologists to score embryo morphology correctly with minimum subjectivity and with high intra and inter-observer agreement is dependent upon competence (i.e., skills and training), accuracy and consistency. The interpretation skills could be achieved through constant education, training and validation of operator competency, which should therefore be a priority during continuous training of embryologists [40]. Therefore, a standardized scoring for embryo quality is important for classification and selection task prior to embryo implantation process. As an example, the addition of viability markers has been proposed to increase the possibility of standardized measurements in embryo morphology scoring [40], and further evaluation on blastocyst quality using quantitative methods has been suggested to quantify morphologic parameters [27].  
Traditionally, morphology-based grading has been approached in attempt to evaluate and select good quality embryos for transfer. Other approaches of preimplantation genetic testing ((PGT)-based method [42]) have been proposed and are becoming popular as principal modalities for embryo selection. Furthermore, the technological advancements in biomedical and bioinformatics fields sees a significant effort focused into developing techniques for analyzing microscopic embryo images particularly at the blastocyst stage.