Background The authors previously reported an increased risk of hospitalisation for acute lower respiratory infection up to age 2 years in children delivered by elective caesarean section. In view of increasing rates of elective caesarean delivery, this association warranted further investigation.

Objective To examine associations between the number of hospital admissions for bronchiolitis and pneumonia and elective caesarean delivery.

Design Retrospective population-based data linkage cohort study of 212 068 non-Aboriginal singleton births of 37–42 weeks gestation.

Methods Negative binomial regression was used to examine associations between mode of delivery and hospitalisations for both bronchiolitis and pneumonia in children aged <12 months and 12–23 months. Models were adjusted for obstetric and known risk factors.

Results 16% of children were delivered by elective caesarean section (n=33 421). In adjusted analysis, compared with spontaneous vaginal delivery, these children had increased risk of admissions for bronchiolitis at age <12 months (incidence rate ratio (IRR) 1.11; 95% CI 1.01 to 1.23) and 12–23 months (IRR 1.20; 95% CI 0.94 to 1.53) independent of other fetal and maternal factors. There was no association between elective caesarean delivery and number of pneumonia admissions aged <12 months (IRR 1.03; 95% CI 0.80 to 1.33) and 12–23 months (IRR 1.09; 95% CI 0.88 to 1.34).

Conclusion Delivery by elective caesarean was independently associated with infant admissions for bronchiolitis but not pneumonia. Elective caesareans or delivery without labour may result in impaired immunity in the newborn leading to increased risk of early viral lower respiratory infections.