The association of obstetric anal sphincter injury and mediolateral episiotomy with increasing birth weight and duration of second stage of labour in spontaneous vaginal delivery

Objectives

Analysis of the association of mediolateral episiotomy (MLE) with obstetric anal sphincter injury (OASI) in women with spontaneous vaginal delivery.

Design.

Population-based cohort study with data from the Netherlands Perinatal Registry, describing 541 055 women who delivered a singleton live born infant in cephalic presentation spontaneously at term. Risk indicators for OASI were tested using univariate and multivariate analysis. Additional analysis for the interaction of MLE with other risk indicators was performed.

Results

The rate of OASI was 4.2 % in 215 241 nulliparous and 1.4 % in 325 814 multiparous women. In nulliparous and multiparous women MLE was associated with a reduction of OASI (adjusted OR (aOR) 0.3, 95 % CI 0.30–0.34 and aOR 0.32, 95 % CI 0.30–0.34).

The association of MLE with a reduced rate of OASI was stronger in high birthweight and in prolonged 2nd stage groups.

In nulliparous women, the number needed to treat (NNT) for the use of MLE to prevent one OASI is 31 in general. With MLE, the OASI rate reduced from 11.5 % to 2.9 with a NNT of 12 in the group with a birth weight ≥ 4000 g and a duration of the second stage of labour of 60–120 min. The NNT is 9 In the group with a birth weight ≥ 4000 g and a duration of the second stage of labour ≥ 120 min (reduction rate of OASI from 14.2 % to 3.5 %).

Conclusions

Use of MLE is associated with a reduction of OASI in spontaneous vaginal delivery.

In nulliparous women, an episiotomy with an anticipated birth weight > 4000 g and a duration of the 2nd stage of more than 60 min should be considered.