Medline ® Abstract for Reference 28

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 episiotomywas 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.