Background Pregnancy-related morbidity and mortality is much more prevalent among adolescents than adults. Adolescent pregnancy is therefore a significant public health problem. Most births to adolescents (95 ) occur in resource-constrained countries. Objective The aim was to review the available evidence about the factors associated with adolescent pregnancy in low-income and lower middle-income countries. Methods The review used the PRISMA procedure of identification, screening and eligibility of publications. PubMed, OVID MEDLINE, SCOPUS and CINAHL plus were searched systematically for peer-reviewed English language papers published before December 2013. Findings In total, 2005 articles were identified and 12 met the inclusion criteria and were reviewed. Despite varied methods, there was substantial consistency in the findings. Limited education, low socioeconomic position, insufficient access to and non-use of contraception were consistently found to be risks for pregnancy among adolescents. There was some evidence that early marriage, living in a rural area, early sexual initiation, belonging to an ethnic and religious minority group also increased the risk of adolescent pregnancy. Higher education, access to income-generating work and family support were found to protect against adolescent pregnancy. Conclusions In resource-constrained countries, as in well-resourced countries, low socioeconomic position appears to increase the risk of pregnancy among adolescents. Additional risks specific to these contexts include cultural traditions such as early marriage and inaccurate beliefs about contraception. It is unlikely that strategies to reduce pregnancy among women aged less than 20 years will be effective unless these are addressed directly.