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: <17 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 <17 years and 17–19 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 <32 weeks (aOR 1.66 (1.10 to 2.51) and 1.20 (1.04 to 1.38)). Women with advancing age (≥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.