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Background  Obstetric anal sphincter injury (OASI) is a rare but serious outcome of vaginal birth. Based on concerns about the increasing number of women who commence childbearing later than previous generation, this study aimed at investigating age-related risk of OASI in women of different parity.    Methods  A population-based register study including 959,559 live singleton vaginal births recorded in the Swedish Medical Birth Register 1999 to 2011. In each parity group risks of OASI at age 25–29 years, 30–34 years, and ≥35 years compared with age < 25 years were investigated by logistic regression analyses, adjusted for year of birth, education, region of birth, smoking, Body Mass Index, infant birthweight and fetal presentation; and in parous women, history of OASI and cesarean section. Additional analyses also adjusted for mediating factors, such as epidural analgesia, episiotomy, and instrumental delivery, and maternal age-related morbidity.    Results  Rates of OASI were 6.6%, 2.3% and 0.9% in first, second and third births respectively. Age-related risk increased from 25-29 years in first births (Adjusted OR 1.66; 95% CI 1.59–1.72) and second births (Adjusted OR 1.78; 95% CI 1.58–2.01), and from 30-34 years in third births (Adjusted OR 1.60; 95% CI 1.00–2.56). In all parity groups the risk was doubled at age ≥ 35 years, compared with the respective reference group of women under 25 years. Adding mediating factors and maternal age-related morbidity only marginally reduced these risk estimates.    Conclusion  Maternal age is an independent risk factor for OASI in first, second and third births. Although age-related risks by parity are relatively similar, more nulliparous than parous women will be exposed to OASI due to the higher baseline rate.    Electronic supplementary material  The online version of this article (10.1186/s12884-017-1473-7) contains supplementary material, which is available to authorized users.

Risk of obstetric anal sphincter injury increases with maternal age irrespective of parity: a population-based register study