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

31 March 2016

Objective: To determine whether OASI rates are increasing at equal rates among different vaginal birth modes. Methods: Using New South Wales (NSW) linked population data, the overall yearly OASI rates were determined among the 261,008 primiparous vertex singleton term births which occurred from 2001-2011. OASI rates among non-instrumental, forceps and vacuum births with and without episiotomy were also determined. Multivariable logistic regression was used to ascertain the adjusted odds ratios (aORs) for each birth category by year. The trends of the aORs over time for each birth category were compared. Results: The overall OASI rate was 4.1% in 2001 and 5.9% in 2011. The highest OASI rates were among forceps births without episiotomy (12.2% in 2001, 14.8% in 2011), and lowest for non-instrumental births without episiotomy (2.6% in 2001, 4.4% in 2011). After adjustment for known risk factors, the only birth categories to show significant increases with OASI over the study period were non-instrumental without episiotomy and forceps with episiotomy (linear trend p<0.01). Conclusion: Overall OASI rates have continued to increase. Known risk factors do not fully explain the increase for non-instrumental births without episiotomy and forceps with episiotomy. Changes in clinical management and/or reporting may be contributing.