Trends in obstetric anal sphincter injuries and associated risk factors for vaginal singleton term births in New South Wales 2001–2009

Authors

  • Amanda J Ampt,

    1. Clinical and Population Perinatal Health Research, Kolling Institute of Medical Research, University of Sydney, St Leonards, New South Wales, Australia
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  • Jane B Ford,

    Corresponding author
    • Clinical and Population Perinatal Health Research, Kolling Institute of Medical Research, University of Sydney, St Leonards, New South Wales, Australia
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  • Christine L Roberts,

    1. Clinical and Population Perinatal Health Research, Kolling Institute of Medical Research, University of Sydney, St Leonards, New South Wales, Australia
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  • Jonathan M Morris

    1. Clinical and Population Perinatal Health Research, Kolling Institute of Medical Research, University of Sydney, St Leonards, New South Wales, Australia
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  • The authors confirm that no commercial or other conflicts of interest exist.
  • Financial support for this research was provided by a National Health and Medical Research Council Centre for Research Excellence Grant (#1001066); Dr Ford is supported by an NHMRC Capacity Building Grant (#573122); and Associate Professor Roberts by a NHMRC Senior Research Fellowship (#457078).

Correspondence: Ms Amanda J. Ampt, Clinical and Population Perinatal Health Research, Kolling Institute, The University of Sydney, Building 52 Royal North Shore Hospital, St Leonards NSW. 2065, Australia.Email: amanda.ampt@sydney.edu.au

Abstract

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.

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