Association of oxytocin augmentation and duration of labour with postpartum haemorrhage: A cohort study of nulliparous women

Abstract

Objective Both duration of labour and use of oxytocin for augmentation are known risk factors for postpartum haemorrhage but distinguishing between the significance of these factors is complex. In this study, we aimed to investigate the association between both labour duration and oxytocin augmentation, for postpartum haemorrhage. Design A cohort study based on a secondary analysis of a cluster-randomised trial. Participants and setting Term nulliparous women with a single foetus in cephalic presentation, spontaneous onset of active labour and a vaginal birth. The participants were originally included in cluster-randomised trial conducted in Norway from December 1, 2014, to January 31, 2017, that aimed to compare the frequency of intrapartum caesarean sections when adhering to the WHO partograph versus Zhang's guideline. Measurements The data were analysed through four statistical models. Model 1 investigated the effect of oxytocin augmentation as a dichotomous variable (yes/no); Model 2 investigated the effect of the duration of oxytocin augmentation; Model 3 investigated the effect of the maximum dose of oxytocin; and Model 4 investigated the effect of both the duration of augmentation and the maximum dose of oxytocin. All four models included duration of labour divided into five time-intervals. We used binary logistic regression to estimate the odds ratios of postpartum haemorrhage, defined as blood loss of ≥ 1000 ml, including a random intercept for hospital and mutually adjusting for oxytocin augmentation and labour duration in addition to maternal age, maternal marital status, maternal higher education level, maternal smoking habits in the first trimester, maternal body mass index and birth weight. Findings Model 1 found a significant association between the use of oxytocin and postpartum haemorrhage. In Model 2, oxytocin augmentation of ≥ 4.5 h was associated with postpartum haemorrhage. In Model 3, we found an association between a maximum dose of oxytocin of ≥ 20 mU/min and postpartum haemorrhage. Model 4 showed that a maximum dose of oxytocin ≥ 20 mU/min was associated with postpartum haemorrhage both for those augmented < 4.5 h and for those augmented ≥ 4.5 h. Duration of labour was associated with postpartum haemorrhage in all models if lasting ≥ 16 h. Key conclusions We found both oxytocin augmentation and labour duration to be associated with postpartum haemorrhage. Oxytocin doses of ≥ 20 mU/min and a labour duration of ≥ 16 h showed an independent association. Implication for practice The potent drug oxytocin should be carefully administered, as doses of ≥ 20 mU/min were associated with an increased risk of PPH, regardless of the duration of oxytocin augmentation.