The incidence of and risk factors for a repeat obstetric anal sphincter injury (OASIS) in the vaginal birth subsequent to a first episode of OASIS: a hospital-based cohort study

To identify the incidence of and risk factors for a repeat obstetric anal sphincter injury (OASIS) in women who sustained an OASIS in their first vaginal delivery and have a subsequent vaginal birth. Data were collected retrospectively for women having had singleton cephalic presentation vaginal deliveries between 2007 and 2015. Women with breech deliveries, stillbirths, foetal congenital abnormalities and multiple pregnancies were excluded. Over the study period, we identified 11,191 women who had a first vaginal birth, of which 603 (5.4%) sustained a first episode of OASIS. Of these women, 243 (40.2%) had a subsequent pregnancy with 190 (78.1%) having a second vaginal birth, 13 (5.4%) an emergency caesarean section (CS) delivery while in labour and 40 (16.5%) an elective CS delivery. In those who delivered vaginally, 16 (8.4%) women had a repeat OASIS. After adjusting for several confounding factors, it was found that the risk of a repeat OASIS was associated with the use of epidural analgesia (OR = 3.66; 95% CI: 1.14-11.71) and an episiotomy in the first delivery (OR = 3.93; 95% CI:1. 03-15.02) and a short labour (< 2.8 h) in the second delivery (OR = 14.55; 95% CI: 1.83-115.75). The time interval between the two vaginal births was not associated with any increased risk of a repeat OASIS. We found that 8.4% of women sustained a repeat OASIS in a subsequent vaginal birth with this risk being associated with the presence of a short second labour and certain features from the first labour.