Estimations of attributable risk
Fifty-six percent of the increase in caesarean deliveries (from 19% to 30%) could be attributed to changes in the distribution of maternal factors including maternal age, BMI, parity and history of previous caesarean delivery (Table 4). Over the time-period, the caesarean delivery rate would have increased from 19% to 25.3% based on differences in maternal parity, age, and BMI alone. Sixty-six percent of the increase in caesarean deliveries (from 19% to 30%) was accounted for when changes in caesarean deliveries for malpresentation, multiple gestation, and preterm birth were considered, and 78% was accounted for when planned (pre-labour) caesarean deliveries for maternal choice, suspected fetal compromise, a previous pregnancy issue or a suspected large fetus were excluded from the analysis. This left 22% of the increase in caesarean deliveries unexplained.
Expressed as a proportion of all births, the prevalence of planned caesarean deliveries for: maternal choice increased from 0.20% to 0.64% of all births; suspected fetal compromise increased from 0.33 to 1.36%; a previous pregnancy issue (such as anal sphincter injury, pelvic floor trauma, or difficult birth) increased from 0.0% to 0.66%; suspected large fetus increased from 0.24% to 0.55%.