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.