"Balai Besar Guru Penggerak Daerah Istimewa Yogyakarta (BBGP DIY)" conducted a professional development course aimed at enhancing primary school teachers' capacity to develop and implement STEM lesson plans in their classrooms. As part of the course, teachers were asked to record their lessons, which were later analysed to identify the mathematical content involved and how teachers delivered this content during the STEM lessons. The findings indicated that, although mathematics appeared to be less pivotal than other STEM disciplines in the lesson plans, the STEM activities provided rich opportunities for developing students' mathematical content knowledge. Most of the teachers realised this and were able to deliver the mathematical content within the context of STEM education. This paper presents an insight into how primary school teachers in Indonesia deliver mathematical content in their STEM lessons and highlights the role of mathematics in STEM education.