Rate of spontaneous onset of labour before planned repeat caesarean section at term

Background: Guidelines recommend that, in the absence of compelling medical indications (low risk) elective caesarean section should occur after 38 completed weeks gestation. However, implementation of these guidelines will mean some women go into labour before the planned date resulting in an intrapartum caesarean section. The aim of this study was to determine the rate at which low-risk women planned for repeat caesarean section go into spontaneous labour before 39 weeks. Methods: We conducted a population-based cohort study of women who were planned to have an elective repeat caesarean section (ERCS) at 39-41 weeks gestation in New South Wales Australia, 2007-2010. Labour, delivery and health outcome information was obtained from linked birth and hospital records for the entire population. Women with no pre-existing medical or pregnancy complications were categorized as 'low risk'. The rate of spontaneous labour before 39 weeks was determined and variation in the rate for subgroups of women was examined using univariate and multivariate analysis. Results: Of 32,934 women who had ERCS as the reported indication for caesarean section, 17,314 (52.6%) were categorised as 'low-risk'. Of these women, 1,473 (8.5% or 1 in 12) had spontaneous labour or prelabour rupture of the membranes before 39 weeks resulting in an intrapartum caesarean section. However the risk of labour < 39 weeks varied depending on previous delivery history: 25% (1 in 4) for those with spontaneous preterm labour in a prior pregnancy; 15% (1 in 7) for women with a prior planned preterm birth (by labour induction or prelabour caesarean) and 6% (1 in 17) among those who had only previously had a planned caesarean section at term. Smoking in pregnancy was also associated with spontaneous labour. Women with spontaneous labour prior to a planned CS in the index pregnancy were at increased risk of out-of-hours delivery, and maternal and neonatal morbidity. Conclusions: These findings allow clinicians to more accurately determine the likelihood that a planned caesarean section may become an intrapartum caesarean section, and to advise their patients accordingly.