Objective. To study predictive associations between psychosocial factors at age 8 and becoming a mother under the age of 20. Design: Prospective follow-up study. Setting: Finland. Population. 2867 girls born in 1981. Methods. Information on family background and psychiatric symptoms was collected at age 8. The associations between these factors and becoming a teenage mother were analyzed using logistic regression analysis. Main outcome measures. Data on births by the age of 20 collected from the hospital discharge register. Results. 128 girls (4.8%) had given birth at the age of 15–19 years. Childhood conduct problems and hyperactive problems, having young mother and family structure other than two biological parents had an independent association with becoming a teenage mother. Conclusions. Girls with externalizing type of problems in childhood have an increased risk of becoming teenage mothers. These problems may also complicate their motherhood.