Objectives: To evaluate the associations between maternal age and obstetric and neonatal outcomes in primiparous women with emphasis on teenagers and older women. Design: A population-based cohort study. Setting: The Swedish Medical Birth Register. Participants: Primiparous women with singleton births from 1992 through 2010 (N=798 674) were divided into seven age groups: less than17 years, 17-19 years and an additional five 5-year classes. The reference group consisted of the women aged 25-29 years. Primary outcome: Obstetric and neonatal outcome. Results: The teenager groups had significantly more vaginal births (adjusted OR (aOR) 2.04 (1.79 to 2.32) and 1.95 (1.88 to 2.02) for age less than17 years and 1719 years, respectively); fewer caesarean sections (aOR 0.57 (0.48 to 0.67) and 0.55 (0.53 to 0.58)), and instrumental vaginal births (aOR 0.43 (0.36 to 0.52) and 0.50 (0.48 to 0.53)) compared with the reference group. The opposite was found among older women reaching a fourfold increased OR for caesarean section. The teenagers showed no increased risk of adverse neonatal outcome but presented an increased risk of prematurity less than32 weeks (aOR 1.66 (1.10 to 2.51) and 1.20 (1.04 to 1.38)). Women with advancing age (greater than= 30 years) revealed significantly increased risk of prematurity, perineal lacerations, preeclampsia, abruption, placenta previa, postpartum haemorrhage and unfavourable neonatal outcomes compared with the reference group. Conclusions: For clinicians counselling young women it is of importance to highlight the obstetrically positive consequences that fewer maternal complications and favourable neonatal outcomes are expected. The results imply that there is a need for individualising antenatal surveillance programmes and obstetric care based on age grouping in order to attempt to improve the outcomes in the age groups with less favourable obstetric and neonatal outcomes. Such changes in surveillance programmes and obstetric interventions need to be evaluated in further studies. ; Funding Agencies|County Council of Ostergotland; Linkoping University