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Background: Elective caesarean delivery is increasing rapidly in many countries, and one of the reasons might be that caesarean delivery is widely believed to protect against pelvic floor disorders, including anal incontinence. Previous studies on this issue have been small and with conflicting results. The aim of present study was to compare the risk of developing anal incontinence in women who had a caesarean delivery, in those who had a vaginal delivery, and in two age-matched control groups (nulliparous women and men).

Methods: In this observational population-based study, we included all women in the Swedish Medical Birth Register who gave birth by caesarean delivery or vaginal delivery during 1973-2015 in Sweden and were diagnosed with anal incontinence according to ICD 8-10 in the Swedish National Patient Register during 2001-15. Exclusion criteria were multiple birth delivery, mixed vaginal and caesarean delivery, and four or more deliveries. We compared the diagnosis of anal incontinence between women previously delivered solely by caesarean delivery and those who solely had delivered vaginally. We also compared it with two age-matched control groups of nulliparous women and men from the Swedish Total Population Register. Finally, we analysed risk factors for anal incontinence in the caesarean delivery and vaginal delivery groups.

Findings: 3 755 110 individuals were included in the study. Between 1973 and 2015, 185 219 women had a caesarean delivery only and 1 400 935 delivered vaginally only. 416 (0.22 %) of the 185 219 women in the caesarean delivery group were diagnosed with anal incontinence compared with 5171 (0.37%) of 1 400 935 women in the vaginal delivery group. The odds ratio (OR) for being diagnosed with anal incontinence after vaginal delivery compared with caesarean delivery was 1 center dot 65 (95% CI 1 center dot 49-1 center dot 82; p<0.0001). When the combination vaginal delivery and caesarean delivery was compared with the nulliparous control group, the OR of being diagnosed with anal incontinence was 2 center dot 05 (1 center dot 92-2 center dot 19; p<0.0001). For the nulliparous women compared with men, the OR for anal incontinence was 1 center dot 89 (1 center dot 75-2 center dot 05; p<0.0001). The strongest risk factors for anal incontinence after vaginal delivery were high maternal age, high birthweight of the child, and instrumental delivery. The only risk factor for anal incontinence after caesarean delivery was maternal age.

Interpretation: The risk of developing anal incontinence increases after pregnancy and delivery. Women with known risk factors for anal incontinence should perhaps be offered a more qualified post-partum examination to enable early intervention in case of injury. Further knowledge for optimal management are needed. Copyright (c) 2019 Elsevier Ltd. All rights reserved.