Background: It is estimated that 15 million births occur every year to adolescents. This represents about 11%of all births each year. The global average rate of births per 1000 females aged 15-19 years is 65. A range of social, cultural, biological and service delivery factors contribute to the high levels of adolescent pregnancy and childbirth.This study was conducted to determine the causes of adolescent pregnancy.Materials and Methods: By searching more than 35 related articles in WHO, Google, Science direct and Pubmed, reference books and by using key words of pregnancy, Adolescens, factors contributing this study had been done.Results: The study showed that pregnant teenagers are involved in causes that include: 1. Declining age of menarche, 2. longer periods of education and delayed marriage for some course and delayed marriage adolescents, 3. forced early marriage and pregnancy, 4. initiation of sexual activity during Adolescence, 5. sexual coercion and rape, 6. education level, 7. socio-economic factors, 8. other risk behaviors, 9. lack of knowledge and 10. lack of access to service centers are.Conclusion: Considering that approximately 21% of Iran's population (15 million) are young and High risk behavior such as early and unprotected sexual activity, alcohol use, smoking and drug addiction among teenagers is rising, a quick Planning and precise and serious policy in this area seems necessary.