Obstetric anal sphincter injury rates among primiparous women with different modes of vaginal delivery

OBJECTIVE: To determine whether rates of obstetric anal sphincter injuries (OASIS) are continuing to increase and whether risk of OASIS according to mode of delivery is constant over time. METHODS: In a retrospective population-based study, data were obtained for vaginal singleton vertex deliveries at 37-41 weeks of pregnancy among primiparous women in New South Wales, Australia, between January 2001 and December 2011. Annual OASIS rates were determined among non-instrumental, forceps, and vacuum deliveries with and without episiotomy. Multivariable logistic regression was used to determine adjusted odds ratios for each delivery mode category by year. Trends in adjusted odds ratios over time for each delivery category were compared. RESULTS: OASIS occurred in 955 (4.1%) of 23 081 deliveries in 2001 and 1487 (5.9%) of 25 081 deliveries in 2011. After adjustment for known risk factors, the only delivery categories to show statistically significant increases in OASIS over the study period were non-instrumental deliveries without episiotomy (linear trend P<0.001) and forceps deliveries with episiotomy (linear trend P=0.004). CONCLUSION: Overall, OASIS rates have continued to increase. Known risk factors do not fully explain the increase in OASIS rates in non-instrumental deliveries without an episiotomy and in forceps deliveries with an episiotomy.