The aim of this study is to examine the association between bullying behaviour at the age of 8 and becoming a mother under the age of 20. This birth cohort study included 2,867 Finnish girls at baseline in 1989. Register-based follow-up data on births was collected until the end of 2001. Information, both on the main exposure and outcome, was available for 2,507 girls. Both bullies and victims had an increased risk of becoming a teenage mother independent of family-related risk factors. When controlled for childhood psychopathology, however, the association remained significant for bullies (OR 2.2, 95% CI 1.2-4.1) and bully-victims (OR 1.8, 95% CI 1.05-3.2), but not for pure victims. Reports of bullying and victimisation from the girls themselves, their parents and their teachers were all associated with becoming a teenage mother independent of each other. There is a predictive association between being a bully in childhood and becoming a mother in adolescence. It may be useful to target bullies for teenage pregnancy prevention.