3.6 Major Perineal injury (≥ 3rd degree perineal injury)
A total of 27 women out of the 745 women who had a vaginal birth (3.6%) sustained a major perineal injury. Due to the low number of events, categorical covariates in the multivariable logistic regression were condensed into fewer categories (Table S1). Parity, country of origin (surrogate for ethnicity), use of forceps and birthweight were included in the model as they are recognised risk factors for obstetric anal sphincter injury. Volume of IV fluids was associated with major perineal injury in the univariable regression (ORunadj 2.5; 95% CI: 1.07, 5.9). However, in the multivariable logistic regression, only the use of forceps was associated with major perineal injury (ORadj 3.0 95% CI: 1.2, 7.6) (Table S1).