RESULTS
Our state-wide cohort included 455,455 births. After exclusions, there
were 129,964 births for analysis: 123,535 primiparous women who birthed
vaginally and 5,429 women who birthed vaginally for the first time after
one previous caesarean section (Figure 1). Vaginal birth for both groups
included those who birthed with the assistance of ventouse or forceps.
There were no missing data on VBAC status (exposure); 11.1% of the
entire cohort had missing perineal tear data (outcome). There was no
difference in distribution of missing outcome data
(3rd or 4th degree tears) between
exposure groups (10.1% in controls vs 9.3% in cases; p=0.09).
Imputation of missing outcome data was not performed (as no auxiliary
variables) and so all cases with missing outcome data were excluded from
final analysis. There were 116,047 births remaining for analysis. The
distribution of covariates, including missingness, is presented in Table
1.
Compared with primiparous women having a vaginal birth, women having a
VBAC were more likely to be older (>30 years,
p<0.001), to have a slightly higher body mass index (mean 25.4
vs 24.7, p<0.001), to give birth between 38 and 40 completed
weeks (p<0.001), to have a ventouse birth (23.6 vs 21.3%,
p<0.001) and for their baby to have a higher birthweight (mean
birthweight 3452.7 vs 3377.3g, p<0.001; incidence of
birthweight ≥4000g 10.8 vs 8.2%, p<0.001) (Table 1). The VBAC
group were less likely to have a spontaneous (unassisted) vaginal birth
(56.7 vs 59.1%, p<0.001) and less likely to have an
intrapartum epidural (30.2 vs 36.0%, p<0.001). There was no
difference between the exposure groups in the likelihood of having an
episiotomy (45.0 vs 46.2%, p=0.18).
Women having a VBAC were significantly more likely than primiparous
women to sustain a 3rd or 4th degree
tear during vaginal birth (7.1 vs 5.7%, p<0.001). A
sub-analysis examining only women who had a VBAC demonstrated that those
who sustained a 3rd or 4th degree
tear were more likely to have had a forceps birth (31.3 vs 18.8%,
p<0.001) and a baby with a birthweight ≥4000g (15.9 vs 10.4%,
p<0.001), and less likely to have had an episiotomy (33.6 vs
45.9%, p=0.001) (Table 2).
Unadjusted analysis produced a relative risk for women having a VBAC of
3rd or 4th degree tear of 1.24
(95%CI 1.12 to 1.38) and a risk difference of 1.39% (95%CI 0.66 to
2.12). The regression adjustment estimates were pooled over the 20
imputed datasets providing an adjusted relative risk amongst cases of
1.21 (95%CI 1.07 to 1.38) and a risk difference of 1.22% (95%CI 0.35
to 2.1). Sensitivity analysis was performed on complete case cohorts,
which produced similar adjusted relative risks of 1.19 (95%CI 1.03 to
1.38) when both BMI and analgesia were included, and 1.23 (95%CI 1.09
to 1.39) when BMI and analgesia were both excluded (Table 3).