Background: Despite the increasing need for STEM skills, to date, the connection between STEM subject choices and their impact on students' educational pathways has not been widely studied. Focusing on the mathematics choice (basic/advanced/no mathematics), a large register dataset that covered students admitted to Finnish universities during 2013-2015 (N = 46,281) was combined with upper-secondary school matriculation examination data (N = 93,955) to find out how this choice influenced the students' university admissions. This large dataset was also examined to establish the current gender distributions in different university degree programs from the perspective of mathematics choices. Further, to find out the students' reasons behind their mathematics choices, a cohort sample of 802 student responses was collected from upper-secondary schools. We also investigated the students' interests in different fields of study to establish any gender differences in them. Results: The register data analysis suggested that in Finland, students' mathematics choices had a strong influence on the university admission outcomes. For instance, only 33% of the upper-secondary school graduates took the advanced mathematics ME test in 2013-2015, yet the number of those admitted to universities who had taken the advanced mathematics ME test was 55%. Most of the university degree programs were female dominated, yet the university students with advanced mathematics were mostly male, and especially the STEM fields in the Finnish universities were male dominated. As for the reasons behind the mathematics choices, students who chose advanced mathematics believed in its usefulness for their future studies and careers. We also found significant gender-based educational differences regarding all the study fields, with STEM careers attracting more males than females. Conclusion: Advanced mathematics was highly valued in Finnish universities, and many students chose advanced mathematics believing in its usefulness for their future studies or careers. Yet, their further study interests and career plans were segregated by gender. As there is a rising need for STEM skills, we must seek effective ways to deliver the evolving possibilities of STEM fields to students, especially girls, during the earlier years of their educations.