Background
Changes in clinical practice and in the characteristics of childbearing women have the potential to influence the rate of obstetric anal sphincter injuries (OASIS). To date, little investigation has been undertaken to assess the effect of risk factor trends for the Australian population on OASIS rates.

Aims
To ascertain the OASIS rates amongst singleton vaginal births ≥37 weeks gestation in NSW, 2001 – 2009; to determine risk factor effect sizes and trends; and to compare predicted with observed OASIS rates.

Methods
Using two linked population‐based data sets, risk factors for OASIS were determined by logistic regression. Contingency tables and predictive modelling were used to determine trends and predicted rates of OASIS, respectively.

Results
The OASIS rate increased from 2.2% in 2001 to 2.9% in 2009. Highest risks were for forceps deliveries without episiotomy (primiparas aOR 6.10, multiparas aOR 6.15), followed by multiparas with no previous vaginal birth (aOR 5.61). High birthweight, vacuum delivery and Asian country of birth posed risks for all women. The greatest risk factor trends were increases in Asian country of birth and vacuum delivery, while the greatest trend amongst protective factors was an increase in maternal age ≥35 years for primiparas. Predicted OASIS rates were lower than observed rates.

Conclusion
In an environment of changing demographic and clinical risk factors, the OASIS rate has increased. This increase is only minimally explained by the identified risk factors and may be related to other unmeasured risk factors or a possible increase in clinical ascertainment and/or documentation of OASIS.