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).