Statistical analyses
Data were analysed using SAS version 9.4. Proportions were expressed as
percentages. Non-normally distributed continuous data were expressed as
medians and interquartile ranges. We described differences in raw
caesarean delivery rates between Group A and Group B by Robson group.
Robust Poisson regression was used to generate relative risks for the
outcome of caesarean delivery, using the following explanatory
variables: time-period (Group A or Group B), maternal age, maternal body
mass index, parity group (nulliparous, parous, or parous with ≥ 1
previous caesarean deliveries). Due to interactions between plurality,
malpresentation, and preterm birth with time-period, the data were
divided into four strata: (1) term cephalic singleton births; (2)
preterm singleton births; (3) multiple gestations; (4) term non-cephalic
singleton births. Robust Poisson regression was performed only in the
first stratum (term, cephalic, singleton births) because maternal
characteristics were not associated with the increase in caesarean
deliveries within the other three strata.
Linearity of maternal age and BMI was assessed by categorizing these
variables in to four groups and plotting the β-coefficients in the
regression equation against the midpoints of these groups. If there was
a non-linear relationship, the categorized variables were used. Missing
data were handled using multiple imputation with 20 iterations.
To estimate the contribution of maternal factors to changes in the
caesarean delivery rate within the first stratum ([1] term cephalic
singleton births), we calculated the increase in number of caesarean
deliveries attributable to maternal factors using the following formula:
\begin{equation}
{Increase\ in\ number\ of\ caesarean\ deliveries\ attributable\ to\ maternal\ factors=\backslash n}{Increase\ in\ number\ of\ caesarean\ deliveries\ among\ women\ \times\ Proportion\ of\ the\ increase\ in\ the\ caesarean\ delivery\ rate\ attributed\ to\ maternal\ factors\backslash n}\nonumber \\
\end{equation}Where,
\begin{equation}
Increase\ in\ number\ of\ caesarean\ deliveries\ among\ women=\ \left(P_{B}-P_{A}\right)\times N_{B}\nonumber \\
\end{equation}where Pi = proportion of all women with a term
cephalic singleton birth in Group i who had a caesarean delivery, and