Background Obstetric anal sphincter injury (OASI) has steadily increased worldwide in recent years. There has been controversy over water birth and the midwifery practise of “hands off” technique contributing to the increase of OASI injury. Chesterfield Hospital has a high water birth rate (12%) with a rising incidence of OASI. Hence this audit was carried out to identify the possible causes in this rise including water birth as a risk factor.

Methods This retrospective audit was performed over period of 13 months. The standard used was the expected incidence of OASI was 1% of all vaginal deliveries (RCOG green-top guidelines). The aspects reviewed included mode of delivery, birth weight, parity and if episiotomy was performed.

Results The incidence of OASI was 4.5% of all vaginal deliveries. There were 2 missed cases of OASI. Among the OASI cases, 32% laboured in the pool as analgesia or had a water birth. 62% had a normal vaginal delivery, 14% had a water birth and 24% had an instrumental delivery. Majority had a normal fetal weight (82%). 88% has no episiotomy performed. Patients who had a water birth were more likely to sustain an OASI compared to normal vaginal delivery (5.47% water births versus 3.66% normal delivery).

Conclusion The increased incidence of OASI in the unit may be due to the increased detection rate by obstetricians. Water birth is a potential risk factor for OASI but this area needs further research.