Objective To determine the incidence and risk factors of recurrent anal sphincter rupture (ASR).Design Population-based retrospective cohort study.Setting Data were taken from the National Medical Birth Registry, Denmark.Population Patients with a first and a second vaginal delivery in the time period 1997-2010.Methods Univariate analysis and multivariate logistic regression were used to determine risk factors of recurrent ASR.Main outcome measures The incidence of recurrent ASR and odds ratios for possible risk factors of recurrent ASR: age, body mass index, grade of ASR, birthweight, head circumference, gestational age, presentation, induction of labour, oxytocin augmentation, epidural, episiotomy, vacuum extraction, forceps, shoulder dystocia, delivery interval and year of second delivery.Results Out of 159 446 women, 7336 (4.6%) experienced an ASR at first delivery, and 521 (7.1%) had a recurrent ASR (OR 5.91). The risk factors of recurrent ASR in the multivariate analysis were: birthweight (adjusted OR, aOR, 2.94 per increasing kg, 95% CI 2.31-3.75); vacuum extraction (aOR 2.96, 95% CI 2.03-4.31); shoulder dystocia (aOR 1.98, 95% CI 1.11-3.54); delivery interval (aOR 1.08 by year, 95% CI 1.02-1.15); year of second delivery (aOR 1.06, 95% CI 1.03-1.09); and prior fourth-degree ASR (aOR 1.72, 95% CI 1.28-2.29). Head circumference was a protective factor (aOR 0.91 per increasing cm, 95% CI 0.85-0.98).Conclusions The incidence of recurrent ASR was 7.1%. Risk factors of recurrent ASR were excessive birthweight, vacuum extraction, shoulder dystocia, delivery interval, year of second delivery and prior fourth-degree ASR. A larger head circumference reduced the risk of recurrent ASR.
