This paper explores the nature of prospective teachers' noticing of students' understanding as they analyze and discuss middle school students' understandings of trapezoids in micro-case videos in the context of geometry. In this exploratory study, the data were obtained from eight prospective middle school mathematics teachers through individual video analysis, reflection papers, and group discussions. The results indicated that the use of purposeful micro-case video designs based on prospective teachers' background knowledge of quadrilaterals allowed them to be productive in video analyses and discussions. In individual video analyses, prospective teachers attended to various mathematical elements to identify students' responses but did not always use them to make interpretations of each student's understanding of trapezoid. In the group discussions of the micro-case videos, in contrast, prospective teachers could provide alternative interpretations of students' understanding by identifying links between the mathematical elements in students' responses and the characteristics of students' understandings. In the group discussions, they provided more detailed and specific instructional actions to support each student's understanding of trapezoid than their individual video analyses. This study suggests practical implications for teacher education programs on how to use video cases (e.g., firstly, working individually and then having group discussions about the videos) to explore prospective teachers' professional noticing skills. Considering prospective teachers' background knowledge of related mathematical contents, this study can also inspire future studies on how to design effective videos about students' mathematical understanding.