Vacuum-assisted deliveries and the risk of obstetric anal sphincter injuries-a retrospective register-based study in Finland

Objectives: To identify and quantify the risks of obstetric anal sphincter injury (OASIS) separately in nulliparae, including women admitted for a first vaginal delivery after a previous caesarean section for their first birth, and multiparae delivered by vacuum extraction in Finland where the type of episiotomy is exclusively lateral.

Design: A retrospective population-based register study.

Methods: Nulliparous and multiparous women with OASIS were compared separately with women without OASIS using stepwise logistic regression analysis.

Main outcome measures: Risk of OASIS.

Results: Among a sample of 16,802 women whose infants were delivered by vacuum extraction between 2004 and 2007, the incidence of OASIS was significantly higher among nulliparous women (475 of 13,981, 3.4%) than multiparous women (40 of 2821, 1.4%), with adjusted odds ratio 2.44 (95% CI 1.77-3.39). Lateral episiotomy was associated with 46% decreased incidence of OASIS (adjusted odds ratio 0.54, 95% CI 0.42-0.70) in nulliparae delivered by vacuum extraction. There was no statistically significant association for multiparous women. An increase of 1000 g in birthweight increased the OASIS incidence 2.10-fold for nulliparae and 2.83-fold for multiparae.

Conclusions: Nulliparous women with infants delivered by vacuum extraction had an increased risk of OASIS compared with multiparous women. Lateral episiotomy was associated with a decreased incidence of OASIS, especially in women with large babies and long second stage. These results support liberal use of lateral episiotomy at vacuum extraction for nulliparous women at high risk of OASIS, but the role of episiotomy should be re-investigated in interventional randomised trials.