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).