Modifiable risk factors of obstetric anal sphincter injury in primiparous women: a population-based cohort study.

Objective: To determine modifiable risk factors and incidence of obstetric anal sphincter injury (OASIS) in primiparous women.
Study Design: We performed a population-based retrospective cohort study, using data from the Danish Medical Birth Registry. The population consisted of primiparous women with a vaginal delivery in the time period 2000-2010. Univariable and multivariable logistic regressions were used to determine risk factors of OASIS. Main outcome measures were incidence of OASIS in first vaginal delivery, odds ratios for possible risk factors: age, body mass index, birthweight, head circumference, gestational age, presentation, induction of labor, oxytocin augmentation, epidural, mediolateral episiotomy, vacuum extraction, forceps, shoulder dystocia, and year of delivery.
Results: Of 214,256 primiparous women with a vaginal delivery, 13,907 (6.5%; 95% confidence interval [CI] 6.4-6.6%) experienced an OASIS. The incidence of OASIS increased in the time period (adjusted odds ratio [aOR], 1.02; 95% CI, 1.02-1.03; P < .0001, per year). We found a protective effect of epidural analgesia (aOR, 0.84; 95% CI, 0.81-0.88; P = .0001). Vacuum extraction without episiotomy was a significant risk factor of OASIS (aOR, 2.99; 95% CI, 2.86-3.12; P < .0001), and episiotomy was protective in vacuum-assisted deliveries compared with vacuum-assisted deliveries without episiotomy (aOR, 0.60; 95% CI, 0.56-0.65; P < .0001). Birthweight was found to be an important nonmodifiable risk factor (aOR, 2.76; 95% CI, 2.62-2.90; P < .0001).
Conclusion: Epidural analgesia in itself was protective against OASIS. Vacuum extraction increased the risk of OASIS, although mediolateral episiotomy was protective when applied in deliveries assisted by vacuum extraction.
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