BACKGROUND: Pregnancy in adolescents is a worldwide health problem and has been mostly common in poor populations. It is not clear if socioeconomic or biological factors are the main determinants of perinatal adverse outcomes in pregnant adolescents. Adolescents under 15 years old may present a high growth rate which may contribute to impair fetal growth. Our aim is to compare perinatal characteristics among early (aged 10 to 14 years) and late (aged 15 to 19 years) pregnant adolescents. METHODS: A cross-sectional study was performed using data from Pernambuco State 2009, obtained from DATASUS/SISNAC, a Brazilian Government, open-access public health database. Maternal and neonatal outcomes were compared between early (aged 10-14 years) and late (aged 15-19 years) pregnant adolescents. Family income was compared between early and late pregnant adolescents using a sample of 412 subjects evaluated at Instituto de Medicina Integral Prof. Fernando Figueira (IMIP) during 2011. Statistical comparisons were made using the chi-square test was used with a significant level of 0.05; bivariate and multivariate analysis were performed. This project was approved by the Institutional Ethics Review Board. RESULTS: Data from 31,209 pregnant adolescents were analyzed. 29,733 (95.2%) were aged 15 to 19 years and 1,476 (4.7%) were aged 10 to 14 years. There were significant differences with respect to marital status, education level and number of prenatal visits of mothers aged 10 to 14 years compared to 15 to 19 years. Of importance, early adolescents had a greater rate of neonates born premature and with low birth weight. Prematurity and low birth weight remained statistically significant after multivariate analysis. CONCLUSIONS: Early aged adolescents may have an increased risk of prematurity and low birth weight. These findings highlight the potential role of biological factors in newborn outcomes in pregnant adolescents.