Maternal and perinatal data were collected by the midwife attending every birth in the state of Victoria, Australia, of at least 20 weeks’ gestation, or, if gestation was unknown, of at least 400 g birth weight. These were reported using a standard perinatal statistics form to the Victorian Perinatal Data Collection (VPDC), which collects this information on behalf of the Consultative Council on Obstetric and Paediatric Mortality and Morbidity (CCOPMM).
The perinatal form contained information on maternal age, parity, gestation, presentation, plurality, birthweight, sex of the baby, pre-existing maternal medical conditions, complications of pregnancy, onset of labour, indication for induction, agents or techniques used to induce or augment labour, analgesia used, method of birth, public or private admission status, as well as many other variables not used in this analysis.
The accuracy of the VPDC items used in these analyses was previously validated on a random sample of cases in the 2003 data set .
Use of the data for the calendar years 2000–2005 inclusive for this project was approved by the Consultative Council on Obstetric and Paediatric Mortality and Morbidity, and the project was approved by the Faculty of Health Sciences, Ethics in Human Research Committee, La Trobe University (approval number FHEC03/146).
In order to compare women with similar levels of risk and no medical indication for induction, according to whether they had labour induced or augmented or a spontaneous onset of labour, cases that did not meet the definition of the “standard primipara” were excluded. For the purpose of this study, a “standard primipara” was defined as being between 20 and 44 years of age, having no complications of pregnancy and no pre-existing diabetes, hypertension, heart disease or mental illness, giving birth for the first time, to a singleton, cephalic-presenting infant with a birthweight between the 10th and 90th centile for sex and gestation, at 37–40 weeks’ gestation. The standard local definition of standard primipara excludes women aged 35 years or older, but given the increasing maternal age at first birth, we extended the definition to include women aged 35–44 years who otherwise met the criteria for the standard primipara. The upper gestational limit was set at 286 days (40 weeks and 6 days) because induction for post-dates at 41 weeks is standard practice in many settings. In order to clarify that we have made these modifications to the local standard primipara definition, we have enclosed the words “standard primipara” in quotes. All births to “standard primiparae” in Victoria from January 2000-December 2005 were included in the study.
Age, parity, plurality, presentation, birthweight and gestation were reported on the perinatal form and entered verbatim into the dataset. The specified maternal medical conditions, as well as pre-eclampsia, gestational diabetes mellitus, pre-labour rupture of the membranes, placental abruption and ‘other APH’ (antepartum haemorrhage) were reported by midwives by ticking a box on the form. The ICD10 code for each of these conditions appeared on the form and was entered into the dataset. Other complications of pregnancy, and indication for induction of labour, were reported as free text in response to the fields ‘other obstetric complications’ and ‘specify indication for induction’ on the form. This text was coded by a health information manager according to ICD10, and the code entered. Up to ten complications of pregnancy could be included. One indication for induction was specified. Forms submitted with missing data, including those with no specified indication for an induced labour, were queried with the hospital, and the data obtained and entered. Women with an entry in any of the ten ‘obstetric complications’ fields were excluded from the “standard primipara” set. Women were also excluded if the indication for induction was an obstetric complication or one of the specified maternal medical conditions.
When labour was induced (e.g. with amniotomy), then an oxytocin infusion commenced some time later, this labour was defined as induced, with the later oxytocin considered to be part of the induction process. When labour began spontaneously, and an oxytocin infusion or amniotomy (or both) were performed some time later, this was considered an augmentation of labour. If the membranes ruptured spontaneously and labour did not follow, an oxytocin infusion administered to stimulate labour was considered by VPDC to be an induction of labour, because there had been no spontaneous onset of labour.
Planned caesarean sections were those that had been arranged before the onset of labour (even if they needed to be moved forward when labour began unexpectedly). Unplanned caesareans were those that took place in response to an emergent problem, whether before labour (e.g. cord prolapse) or during labour.
The small number of women with failed induction of labour were classified as unplanned CS before the onset of labour.
Perinatal mortality included babies who were stillborn and those who died within 28 days of birth. Babies with congenital anomalies are not excluded unless the anomaly was reported as a complication of pregnancy, or as the indication for induction of labour.
In order to explore the effect of each method of induction or augmentation separately in multivariate analysis, a new variable was created with mutually exclusive and exhaustive categories for each combination of onset of labour and method of induction or augmentation. These categories were:
spontaneous onset, no augmentation;
spontaneous onset, augmented with amniotomy;
spontaneous onset, augmented with oxytocin infusion;
spontaneous onset, augmented with amniotomy and oxytocin infusion;
labour induced with prostaglandin gel only;
labour induced with amniotomy only;
labour induced with oxytocin infusion only;
labour induced with prostaglandin and amniotomy;
labour induced with prostaglandin and oxytocin infusion;
labour induced with amniotomy and oxytocin infusion;
labour induced with prostaglandin, amniotomy and oxytocin infusion.
Those who had a CS without labour were excluded from multivariate analysis. Descriptive statistics were used to describe the characteristics of the women included in the study. Proportions were compared using Pearson Chi-square and Fisher’s exact tests.
The association between each of these induction/augmentation methods or combination of methods and method of birth was analysed via multinomial logistic regression, adjusting for potential confounders selected a priori. Adjusted relative risk ratios and their 95 % confidence intervals are reported. Maternal age and birthweight were linearly and positively related to CS, so they were included in the model as continuous variables. Gestation, public/private admission status and use of epidural analgesia were included as categorical variables. The very small number of cases with missing data were excluded from multivariate analysis.
Multinomial logistic regression was used because there are three possible outcomes for method of birth – unassisted vaginal birth, operative vaginal birth or CS. This paper presents the results for caesarean compared with unassisted vaginal birth. The Relative Risk Ratio is the output of multinomial logistic regression but is mathematically equivalent to an Odds Ratio.