To investigate the risk for anal sphincter tears (AST) in infibulated women. Design. Population-based cohort study. Setting. Nationwide study in Sweden. Population. The study population included 250 491 primiparous women with a vaginal singleton birth at 37–41 completed gestational weeks during 1999–2008. We only included women born in Sweden and in Africa. The African women were categorized into three groups; a Somalia group, n = 929, where over 95% are infibulated; the Eritrea-Ethiopia-Sudan group, n = 955, where the majority are infibulated, compared with other African countries, n = 1035, where few individuals are infibulated but had otherwise similar anthropometric characteristics. These women were compared with 247 572 Swedish-born women. Methods. Register study with data from the National Medical Birth Registry. Main outcome measures. AST in non-instrumental and instrumental vaginal delivery. Results. Compared with Swedish-born women, women from Somalia had the highest odds ratio for AST in all vaginal deliveries: 2.72 (95%CI 2.08–3.54), followed by women from Eritrea-Ethiopia-Sudan 1.80 (1.41–2.32) and other African countries 1.23 (0.89–1.53) after adjustment for major risk factors. Mediolateral episiotomy was associated with a reduced risk of AST in instrumental deliveries. Conclusion. Delivering African women from countries where infibulation is common carries an increased risk of AST compared with Swedish-born women, despite delivering in a highly technical quality healthcare setting. AST can cause anal incontinence and it is important to investigate risk factors for this and try to improve clinical routines during delivery to reduce the incidence of this complication.