Study design
We performed a retrospective cohort study including 116,047 women who gave birth in the Australian state of Victoria between 2009 and 2014. Validated data were obtained from the Consultative Council on Obstetric and Paediatric Mortality and Morbidity (CCOPMM) for all births in Victoria. CCOPMM is the central agency that collects and validates data on obstetric and perinatal outcomes within the state (19-21). These data are populated by state-wide hospital documentation of outcomes by midwives caring for the patients at the time of birth.
Our cohort included all women who gave birth to a live baby at term gestations (≥37+0 weeks) in Victoria from 1st Jan 2009 to 31st Dec 2014. Women were excluded if they had missing or incomplete gestational age data, missing or implausible birthweight (<500g or >6500g), gave birth pre-term (<37 weeks) or had a stillborn baby (Figure 1). The primary exposure of interest was the first vaginal birth after one previous caesarean section. Women were therefore excluded if they did not fit the inclusion criteria for a) cases: multiparous women having had only one previous caesarean section followed by a vaginal birth, or b) controls: primiparous women having their first vaginal birth. The primary outcome of interest was documented obstetric anal sphincter injury (3rd or 4th degree tear).