Strengths and Limitations
The National Birth Registry of Austria is maintained by obstetric departments, therefore collected information mainly focuses on obstetric and gynaecologic rather than on anaesthesiologic aspects. No information on cervical dilatation or timepoint of the application of EA, as well as a differentiation of duration of birth between first and second stage of labour, is recorded. Further, type of local anaesthetic, dosage and application modality (bolus, patient-controlled EA, programmed intermittent epidural bolus, dural puncture epidural) for EA is not documented, making it difficult to draft assumptions about the impact of EA technique on the altered outcome variables. It is described that epidural related maternal fever occurs frequently and was shown to be associated with worse neonatal outcome. Maternal body temperature is unfortunately not incorporated in the registry, so an association of EA and maternal fever could not be investigated in this analysis. The use of oxytocin for labour augmentation is also not available. Obstetricians may adapt their procedural decision on the presence of adequate pain relief, which could lead to higher rates of instrumental vaginal deliveries and episiotomies. This bias could not be considered in the statistical models. Outcome variables were sufficiently reported, except for BE, which was missing in 23.5%, although, missing data were equally distributed between EA (22.4%) and noEA (23.8%). All hospitals registered for obstetric services in Austria provide data to the National Birth Registry, representing 98% of annual births. The analysed data is therefore close to the basic population of primiparous women with spontaneous vaginal delivery at term, thereby avoiding a meaningful selection bias.