Main findings
Our study found that women having a vaginal birth after one previous caesarean section were 21% more likely than primiparous women having a vaginal birth to sustain a 3rd or 4th degree tear (RR 1.21, 95%CI 1.07-1.38). Given the baseline risk is not low (6.1% in the UK for instance), this increase may be clinically significant. These findings indicate a level of risk that women should be made aware of during antenatal counselling if a VBAC is being considered. Across the different statistical models, the relative risk estimates were similar, all suggesting a significant increase in the incidence of 3rd or 4th degree tears associated with VBAC.
Our findings highlight the characteristics of women attempting a VBAC, and of those at increased risk of 3rd and 4th degree tears. These findings are in keeping with studies from comparable settings around the world. Women having a VBAC in our Victorian cohort were more likely to be older, have an instrumental birth and have a baby with higher birthweight. Those that sustained a 3rd or 4th degree tear during VBAC were more likely to have had a forceps birth, a baby of birthweight ≥4000g and less likely to have had an episiotomy. These factors are all known to increase the risk of 3rd and 4th degree tears (9, 22, 23), with forceps considered a major risk factor for significant perineal injury (24-26).
In 2014, Hehir et al published results of a study from one large tertiary referral hospital in Dublin that concurred with our findings, that women having a VBAC were more likely to be older and have a baby with higher birthweight (consistent with being multiparous), and have an instrumental birth (10). Women who sustained a significant perineal injury during successful VBAC were also more likely to have had a forceps (27). A study conducted in Southampton, UK, between 2004-2014 looked at the maternal, intrapartum and neonatal factors associated with 3rd or 4th degree tear amongst 1,375 secundiparous women having a VBAC (8). They found that advanced maternal age, higher birthweight and an urgent category of first caesarean section were associated with an increased risk of 3rdor 4th degree tear (8). Episiotomy (right mediolateral) was found to be protective, consistent with the findings of ours and other studies (28-31).
Another UK-based study by Jardine et al examined a cohort of 9,993 secundiparous women having a VBAC and excluded any women with missing data (16). Whilst confirming the above findings, this study also suggested that women having a VBAC were more likely to experience a shoulder dystocia at vaginal birth, and that the increased risk of perineal injury following VBAC was restricted to women who had had an emergency primary caesarean section (16).
Other studies have hypothesised why women having a VBAC are at increased risk of 3rd and 4th degree perineal tears. Proposed mechanisms range from the mismatch between more propulsive uterine contractions in the multigravida VBAC cohort, coupled with a ‘nulliparous perineum’ (32, 33), through to relative cephalo-pelvic disproportion as the indication for first caesarean section affecting the passage of the fetus and leading to anal sphincter injury in subsequent births for these women (9, 11, 32). Further, as demonstrated by ours and a number of studies, women attempting a VBAC are more likely to have an operative vaginal birth (10), likely due to concern from the accoucheur about prolonged second stage and the risk of uterine rupture and fetal compromise (8, 34). This lower threshold for operative birth ultimately increases the risk of perineal injury. Irrespective of the mechanism, there is now convincing evidence that VBAC is associated with an increased risk of significant perineal injury, and education and prevention strategies should be directed toward this group.