Obstetric anal sphincter injury rates among primiparous women with different modes of vaginal delivery Type of article: Clinical – full-length Authors:

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. INTRODUCTION Obstetric anal sphincter injuries (OASIs) (third or fourth degree tears) are a complication of vaginal births which can have profound short and long term consequences for women. Of concern, increasing OASI rates have been reported in population-based studies both from Australia [1] and internationally [2, 3]. In order to identify possible reasons for increasing OASI incidence, attention has turned to examining changes in the rates of known risk factors over time. With the marked increase in caesarean sections internationally [4], the current population of women experiencing a vaginal birth may have different characteristics to women who previously birthed vaginally, and as a result may have a different risk profile for OASI occurrence. For example, among vaginal births in Australia the relative percentage of Asian women has increased by 35% from 2001 to 2009, and the relative percentage of instrumental births by 16.0% [1]. Changes in risk factors may also be a reflection of changes within the birthing population in general. Despite changes in the prevalence of known maternal risk factors and clinical practices, they are not the sole drivers of increasing OASI rates [1, 2]. Attempts to produce a risk scoring system for OASI have been problematic with prediction remaining elusive [1, 5]. Although prediction modelling takes into account changing rates of risk factors, it assumes a constant association of risk over time between each risk factor and OASI. However, with possible changes in clinical practice, staff experience and expertise, and/or a change in population characteristics, the strength of individual risk factors may have changed over time. In addition, increases in reported OASIs may be a consequence of improved clinical detection and reporting. Examination of OASI trends over time within risk factor strata may help to detect if there are women whose risk is increasing disproportionately compared to others in different strata. To date, such studies have not adequately taken into account confounding factors [6] or have been undertaken in populations with unusually low OASI rates [7]. With shifts in the proportion of instrumental births at a population level [8, 9], it is plausible that the strength of risk for OASI may be changing among different birth modes. This population-based study using routinely collected linked data aims to determine if the overall trend for OASI is continuing to rise, and if OASI risk for different modes of birth with and without episiotomy are constant over time. MATERIALS AND METHODS The study population consisted of all primiparous women who had a vaginal singleton vertex birth at 37-41 weeks (term) gestation between 2001 and 2011 in New South Wales (NSW), Australia (n=261,008). NSW is the most populous state in Australia, and with over 95,000 births occurring in 2011 contributes to approximately one third of all Australian births [9]. Data for this study were obtained from the NSW Ministry of Health and consisted of two previously validated population data collections; the NSW Perinatal Data Collection (PDC, birth data), and the NSW Admitted Patients Data Collection (APDC, hospital data). The former is a legislated surveillance system of all births in NSW ≥ 20 weeks gestation or ≥400grams birthweight, and contains maternal, demographic, medical and obstetric information including labour, birth and infant condition details. Data are recorded by the attending midwife or doctor. The APDC is a census of all discharges from public and private hospitals and day procedure centres, with administrative and demographic data as well as information regarding diagnoses and procedures coded according to the International Classification of Diseases, Australian Modification (ICD-10-AM) and the Australian Classification of Health Interventions (ACHI). These datasets were longitudinally linked using probabilistic methods by the Centre for Health Record Linkage, with de-identified records provided to researchers. Obstetric anal sphincter injury (OASI) was identified from the hospital data by one or more of the following: ICD-10-AM diagnosis codes for third or fourth degree perineal laceration during delivery (O70.2 or O70.3), or the Australian Classification of Health Interventions procedure code for suture of third or fourth degree tear (16573-00). This combination has been reported as the most reliable method for ascertainment of OASI occurrence in NSW population health data [10]. Recognised risk factor data (maternal age, Asian country of birth, public/private care, hypertension, induction/augmentation, epidural, episiotomy, gestation, type of birth, birthweight and infant sex) were obtained from either one or both data collections based on accuracy