INTRODUCTION
Obstetric anal sphincter injury reflects significant perineal trauma during childbirth and involves injury of the anal sphincter complex (1). While many heal well after primary repair, the impact of this injury for some can be lifelong with long-term symptoms persisting well beyond the birth (2-4). These symptoms include perineal pain, dyspareunia, defecatory dysfunction and faecal incontinence and they can significantly impact a woman’s quality of life and emotional well-being (3, 5).
The incidence of obstetric anal sphincter injury amongst primiparous women varies but is reported to be 5.3% in Australia, 6.1% in the UK and up to 16% in the US (1, 6, 7). Risk factors include nulliparity, advanced maternal age, and intrapartum factors that might plausibly be associated with pelvic trauma such as infant birthweight >4000g and instrumental vaginal birth (8-11).
In most Western jurisdictions, up to 30% of primiparous women give birth via caesarean section (6, 12). For women in their second pregnancy, VBAC is an increasingly desired mode of birth, with successful VBAC associated with a quicker post-partum recovery (13, 14).
However, compared with vaginal birth in primiparous women, there is emerging evidence suggesting successful VBAC is also associated with an increased risk of obstetric anal sphincter injury (9, 10, 15). Past studies in this field have been limited by small cohort sizes, lack of broader generalisability (limited to a single hospital cohort) or outdated methodology (such as a reliance on complete case records only, which may lead to ascertainment bias) (16).
Prior studies have consistently suggested that women having a VBAC are up to 1.4-fold more likely to sustain significant perineal injury compared with their primiparous counterparts (8, 9, 15). And yet, the risk of perineal injury is still only peripherally acknowledged in many professional guidelines (17, 18) and is not routinely included in antenatal counselling of women considering a VBAC. Using a large cohort of women who achieved a VBAC for their second birth and employing advanced statistical methodology, our study aimed to rigorously validate a possible association between VBAC and significant perineal injury.