The aim of this paper is to examine whether the use of a hypothetical learning trajectory as a guide to notice students’ mathematical thinking could improve pre-service teachers professional discourse and enhance pre-service teachers’ noticing. Twenty-nine pre-service primary school teachers participated in a learning environment in which they had to interpret students’ thinking about the fraction concept using a hypothetical learning trajectory as a guide. Results suggest that using a hypothetical learning trajectory as a guide helped pre-service teachers develop a more detailed discourse when interpreting students’ mathematical thinking, enhancing their noticing skill. The enhancement of the skill of noticing, however, was linked to pre-service teachers’ mathematical content knowledge. This research provides teacher educators with resources to help pre-service teachers produce a more detailed professional discourse to attend to the details of students’ answers and their different mathematical levels of thinking in mathematics teacher education programs.