Prospective comparison of obstetric anal sphincter injury incidence between an Asian and Western hospital
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Introduction and hypothesis
Obstetric anal sphincter injury (OASIS) rates are reported to be higher in Asian women living in Western countries than in those living in Asia, but the reasons for the differences remain unclear. The objectives of this study were for a single examiner to prospectively compare OASIS rates in primiparous Asian women in an Asian and Western birth unit and determine potential birth factors that may influence the possible difference in OASIS incidence.
This was a prospective observational study based in Hong Kong, China, and Sydney, Australia, involving primiparous women > 36 weeks gestation of Asian descent undergoing vaginal delivery. A single examiner recorded basic patient demographics, observed all the deliveries at both sites, noting birthing techniques, and then examined the women, including a rectal examination, to determine OASIS incidence.
Seventy births in Hong Kong and 66 in Sydney were studied. The incidence of OASIS was 34% in Sydney and 10% in Hong Kong (p = 0.001). Birthweight, epidural rate, body mass index, and instrumental delivery were higher in Sydney. Episiotomy rates were higher in Hong Kong (59.2% vs. 82.9%; p = 0.007). When comparing OASIS with no-OASIS, perineal length (OR = 0.36, 95% CI 0.17 to 0.76, p = 0.004) and birthweight (OR = 1.14, 95% CI 1.00 to 1.30, p = 0.039) were independent risk factors for OASIS.
The incidence of OASIS in Asian women is significantly higher in a Western than in an Asian setting. In Asian women, perineal length and birthweight can affect the risk of OASIS at the time of vaginal delivery.
KeywordsBirthweight Episiotomy OASIS Perineal length Primiparous
Body mass index
Gestational diabetes mellitus
Obstetric anal sphincter injuries
We acknowledge Prof. Tony Chung, who helped with the conception of the study and cooperation of both hospitals and helped provide funding. We thank CY Lai and all of the midwives who helped with the recruitment of patients at both sites. We also acknowledge Prof. Kate Moore who helped with editing the manuscript.
The study was funded by the OASIS Research Trust Fund (Sydney) and the Academic Department of Obstetrics and Gynaecology (Hong Kong).
Compliance with ethical standards
Conflicts of interest
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