Objective To assess the role of birth position in the occurrence of anal sphincter tears (AST).
Design Observational cohort study.
Setting South Hospital in Stockholm, a teaching hospital with around 5700 births per year.
Population Among all 19 151 women who gave birth at the South Hospital during the study period 2002–05, 12 782 women met the inclusion criteria of noninstrumental, vaginal deliveries.
Methods Data on birth position and other obstetric factors were analysed in relation to occurrence of AST.
Main outcome measure Third- and fourth-degree AST.
Results AST occurred in 449 women (3.5%). The trauma was more frequent in primiparous (5.8%) than in multiparous women (1.7%). The highest proportion of AST was found among women who gave birth in lithotomy position (6.9%), followed by squatting position (6.4%). Logistic regression analyses showed that lithotomy (adjusted OR 2.02, 95% CI 1.58–2.59) and squatting positions (adjusted OR 2.05, 95% CI 1.09–3.82) were associated with a significantly increased risk for AST. Other major risk factors for anal sphincter trauma were primiparity (adjusted OR 3.29, 95% CI 2.55–4.25), prolonged second stage of labour >1 hour (adjusted OR 1.52, 95% CI 1.11–2.10), infant birthweight more than 4 kg (adjusted OR 2.12, 95% CI 1.64–2.72) and large infant head circumference (adjusted OR 1.57, 95% CI 1.23–1.99).
Conclusion Lithotomy and squatting position at birth were associated with an increased risk for AST also after control for other risk factors.