In Indonesian primary schools, sex education is implicitly integrated into various related subjects, such as science, biology, social studies and religion. The technical facts of ovulation and sperm are mentioned in biology, although little or no connection is made between this process and sexual intercourse. By the end of primary school, therefore, children are likely to have a poor understanding of how pregnancy can occur. Given that young girls at this age have already experienced or will soon experience menarche, and given the increasing trend towards delaying the age at marriage in Indonesian society, this lack of knowledge places young people at risk of various negative consequences, including unwanted pregnancy. In this paper, we investigate the level of understanding regarding human reproduction amongst 1762 students attending the last year of primary school, distributed across 32 different schools in Indonesia. Despite the fact that all schools follow a national curriculum, our results reveal widespread variability in the students' comprehension of how pregnancy can occur. In particular, students attending Islamic religious schools and those in less developed provinces of Indonesia appear to have a much poorer understanding of the link between sexual intercourse and conception.