Does water birth affect the risk of obstetric anal sphincter injury? Development of a prognostic model.

INTRODUCTION AND HYPOTHESIS: Obstetric anal sphincter injury (OASI) is a significant complication of vaginal delivery. Water birth has become a popular preference for women giving birth in the UK, however, there is limited data on the risk of OASI following water birth. Our aim was to assess OASI risk in low-risk women giving birth in water without medical intervention compared with on land and to create a prognostic model for OASI prediction. METHODS: This was a retrospective study of 15,734 low-risk women giving birth by spontaneous vaginal delivery between January 2008 and October 2014 in a midwifery-led unit (MLU). Patient factors and delivery data were analysed to identify differences between water and land births. Univariate analysis determined factors that statistically predicted OASI and was then used to create multivariate analysis. Significant multivariate factors were used to create a prognostic model to predict likelihood of OASI. RESULTS: OASI rates were 1.6% on land and 3.3% in water [odds ratio (OR) 2.10, 95% confidence interval (CI) 1.5-2.94). Multivariate analysis confirmed water birth, ethnicity and parity as independent risk factors for OASI (adjusted OR water birth: 1.77 (CI 1.25-2.51). Our prognostic model showed Black and Asian primigravidae following water birth had the highest risk of OASI and white multiparae on land the lowest. CONCLUSION: This study of comparable low-risk women shows an increased risk of OASI following water birth compared with land birth. Use of this prognostic model will help women determine their risk of OASI following birth in water or on land.