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.