Risk factors for anal sphincter tears: the importance of maternal position at birth

To assess the role of birth position in the occurrence of anal sphincter tears (AST).

Observational cohort study.

South Hospital in Stockholm, a teaching hospital with around 5700 births per year.

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.

Data on birth position and other obstetric factors were analysed in relation to occurrence of AST.

Third- and fourth-degree AST.

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).

Lithotomy and squatting position at birth were associated with an increased risk for AST also after control for other risk factors.