Labour dystocia : risk factors and consequences for mother and infant

Abstract: Background: Labour dystocia (prolonged labour) occurs in the active first stage or in the second stage of labour. Dystocia affects approximately 21-37% of nulliparous, and 2-10% of parous women. The condition is associated with increased risks of maternal morbidities, instrumental vaginal deliveries and is the most common indication for a primary caesarean section. The effects of the duration of second stage on neonatal outcomes are still unclear. Dystocia primarily affects nulliparous women, but the risk of recurrence in following labour has not previously been investigated. The aim of this thesis was to elucidate factors influencing the risk of dystocia and the effects of prolonged second stage of labour on neonatal outcomes. Material and Methods: Studies I-IV are population-based cohort studies. The first two are nation-wide, based on the Swedish Medical Birth Register (MBR). In study I, births between 1992 and 2006 were covered and in study II, the corresponding period was 2006 to 2011. The Stockholm-Gotland Obstetric Cohort was used in the third and fourth studies, from 2008 to 2012 in study III and 2008 to 2013 in study IV. Term and post-term singleton pregnancies with infants in cephalic presentation, were studied in all four papers. In study I, 239 953 women who gave birth to their first and second infants, were assessed regarding the risk of recurrence of labour dystocia and mode of second delivery. In study II, the association between use of low-molecular-weight heparin (LMWH) during pregnancy and risk of labour dystocia was examined in 514 875 nulliparous and parous women. Study III and IV included 32 796 and 42 539 nulliparous women, respectively. The associations between duration of second stage (study III and IV), and adverse neonatal outcomes such as low 5-minute Apgar score (Study III), umbilical cord acidosis, birth-asphyxia-related complications and admission to neonatal intensive care unit (NICU) (study IV) were assessed. In addition, the effect of duration of pushing on adverse neonatal outcomes were examined in study IV. Results: The overall risk of recurrence of labour dystocia in second labour was not very high, but there was a substantial risk of recurrence of labour dystocia in women with previous caesarean section. Instrumental vaginal delivery and caesarean section in second labour were not only associated with previous dystocia and mode of delivery but also with fetal and maternal characteristics. LMWH use during pregnancy was not associated with risk of labour dystocia after adjustments for potential confounders. Increasing duration of second stage was associated with increased risk of low 5-minute Apgar score, birth-asphyxia-related complications, and admission to NICU for the infant. Umbilical artery acidosis increased with duration of pushing, but not with duration of second stage. However, the absolute risk differences of most of the adverse neonatal outcomes, were low. Conclusions: Taking individual obstetric and demographic information into account is important in the risk assessments for dystocia and instrumental delivery in second labour. Use of LMWH during pregnancy does not seem to influence the risk of labour dystocia. With increasing duration of second stage and pushing, fetal surveillance is of outmost importance.