Interpretation
Our study has strong clinical relevance. We set out to validate the association between VBAC and significant perineal injury using a large cohort, and have demonstrated with updated methodology that women having a VBAC are 21% more likely than primiparous women to sustain a 3rd or 4th degree tear during vaginal birth. Current antenatal counselling and patient education for women attempting a VBAC typically focusses on the risk of uterine rupture and subsequent fetal complications (17, 18), often without any reference to the increased risk of obstetric anal sphincter injury. In light of our findings, we feel the evidence now convincingly demonstrates an increased risk of significant perineal injury associated with VBAC. We suggest that this risk should be included in the antenatal counselling of women considering a VBAC and be acknowledged in the guidelines of peak professional bodies such as the Royal Australian and New Zealand College of Obstetricians and Gynaecologists and the Royal College of Obstetricians and Gynaecologists.
Our study is well-timed with many Western jurisdictions introducing initiatives to prevent and reduce the occurrence of 3rd and 4th degree tears. Through measures such as timely mediolateral episiotomy and Manual Perineal Protection, the OASI Care Bundle, introduced throughout the UK in 2016-2017, was shown to significantly reduce the risk of 3rd and 4th degree tears (aOR 0.80; 95%CI 0.65-0.98) in participating units (39). In Victoria, Australia, there is a current state-wide initiative to reduce the rate of 3rd and 4th degree tears amongst all women having a vaginal birth (the Better births for women collaborative ), instigated in 2019 by the peak safety and quality institution, Safer Care Victoria (40). Neither the OASI Care Bundle in the UK or the Better Births initiative in Australia identified women having VBAC as a high-risk group, however our findings suggest that these women should be prioritised in such initiatives.
Our intention in pursuing this study was not to dissuade either patients or healthcare providers from supporting women attempting a vaginal birth after caesarean section, but rather to make antenatal counselling more comprehensive, birth management decisions better informed and to help optimise intrapartum care. There are myriad benefits associated with successful, uncomplicated VBAC, some of which include expedited post-birth recovery and improved breastfeeding initiation (41, 42). Vaginal birth after caesarean section also avoids the well-established risks related to repeat caesarean section delivery, such as significant bleeding, development of abdominal adhesions, increased future risk of placenta praevia, disorders of placental adherence and unnecessary iatrogenic preterm birth in the setting of threatened preterm labour (43, 44).