Obstetric Anal Sphincter Injuries – Maternal, Foetal and
Sociodemographic Risk Factors: A Retrospective Register-Based Study
Objective. To determine risk and protective factors of obstetric anal
sphincter injuries (OASIS). Design. A retrospective register-based
observational study. Setting. Sweden. Population. A cohort of 988, 988
singleton term deliveries 2005-2016 were included. Methods. Data from
the Swedish Medical Birth Registry and Statistics Sweden were extracted
to identify cases of OASIS and maternal and foetal characteristics.
Modified Poisson Regression analyses were performed to assess risk
factors. Main outcome measures. Risk ratios for OASIS with 95%
confidence interval associated with maternal and foetal risk factors
were calculated. Results. The rate of OASIS was 3.5% (n=34, 583).
Primiparity (aRR 3.13 95% CI 3.05–3.21), vacuum extraction (aRR 2.79
95% CI 2.73–2.86), forceps (aRR 4.27 95% CI 3.86–4.72) and high
birth weight (aRR 2.61 95% CI 2.50–2.72) were associated with a
significantly increased risk of OASIS. Increasing maternal age and
decreasing maternal height increased the risk of OASIS. Smoking (aRR
0.74 95% CI 0.70–0.79) and low maternal education (aRR 0.87 95% CI
0.83–0.92) were associated with a decreased frequency of reported
OASIS. Obesity decreased the risk of OASIS (aRR 0.90 95% CI
0.87–0.94), but only after adjusting for foetal birth weight. Previous
caesarean section increased the risk of OASIS (aRR 1.41; 95% CI
1.36–1.47). Conclusion. Primiparity, instrumental delivery and high
birth weight increased the risk of OASIS. Risk factors including BMI,
height, age, smoking, maternal education, ethnicity and previous
caesarean section also contribute to the overall risk of OASIS.
Keywords. Obstetric sphincter injuries, risk factors, pregnancy.