Interpretation
These findings are important because the changes in the rate of caesarean delivery over time mirrored Australian national rates,9 and our more detailed institutional data provided insight into the probable causes of the national rate rise. More than 50% of the increase in rates of caesarean delivery was attributable to changes in baseline maternal factors rather than changes in obstetric practice or maternal choices, although these latter are often assumed to be the main drivers of increasing rates of caesarean delivery. We speculate that increases in planned pre-labour caesarean delivery for suspected large-for-gestational-age fetus and suspected fetal compromise are driven by increases in the prevalence and frequency of antenatal ultrasound.
Twenty-two percent of the increase in caesarean deliveries remained unexplained in our analyses. Possible attributable causes not accounted for in this analysis include the increase in caesarean deliveries in Robson group 5 (previous caesarean delivery, term, singleton cephalic) from 70% to 83% over the time-period. This could be due to a shift away from encouraging vaginal birth after previous caesarean delivery but could also be due to growing numbers of women with two or more previous caesarean deliveries, a factor we were unable to examine in our dataset. The marked increase in caesarean delivery in Robson groups 1 and 2A (nulliparous, term, singleton, labouring women) from 12% to 21% deserves scrutiny. Our previous finding in the same study population that caesarean delivery for slow progress in labour increased from 3.4% to 5.5% of all births9 suggests that this group should be the target of future strategies to increase vaginal births.
Our finding that maternal background characteristics contribute significantly to the increase in the rate of caesarean delivery is consistent with the scientific literature. In some settings, it has been estimated that 14% of caesarean deliveries were due to maternal obesity2,8 and 7% were due to gestational weight gain.14 In Australia in 2018, 24.3% of pregnant women were older than 35 years.15 Assuming a relative risk of 2.0 compared with younger women,3 20% of all caesarean deliveries could be attributed to advanced maternal age.16 However, these estimates did not adjust for multiple factors. In the current study, the two largest factors explaining the increase in the caesarean delivery rate from 19% to 30% was the change in distribution of parity and previous caesarean births (Table 3).
We found an increase in primary caesarean delivery rates from 15% to 23%, and in nulliparous caesarean delivery rates from 20% to 30% (Table 2). Coupled with an approximate doubling in Robson category 5 from 5.6% to 10.6%, of all births, this confirms the findings of others that the increase in the caesarean delivery rate is being driven by an increase in primary caesarean deliveries.11
Table 4 can be used to illustrate what the caesarean delivery rate “would have been” in the 1990’s (Group A) had the obstetric population consisted of exactly the same mix of maternal factors that were present in the 2010’s (Group B). For example, had the population in Group A had the same mix of parity, maternal age and maternal BMI then the caesarean delivery rate would have been expected to be 25.4%, compared to 19.1%, with no changes in clinical care. With additional changes in caesarean delivery rates for malpresentation, multiple gestation, malpresentation and preterm birth, it would have increased to 26.5%, and with changes to rates of planned caesarean delivery for the indications listed in Table 4, it would have increased to 27.9%. Only the remaining two percent was unexplained.