Maternal body mass index and oxytocin in augmentation of labour in nulliparous women: a prospective observational study.

OBJECTIVE

To evaluate oxytocin use for augmentation of labour in relation to body mass index (BMI) on admission to the labour ward, focusing on cumulative oxytocin dose and maximum rate of oxytocin infusion during the first stage of labour.

DESIGN

Prospective observational study.

SETTING

Seven hospitals in Sweden.

PARTICIPANTS

1097 nulliparous women with singleton cephalic presentation pregnancy, ≥37 weeks of gestation, spontaneous onset of labour and treatment with oxytocin infusion for labour augmentation. The study population was classified into three BMI subgroups on admission to the labour ward: normal weight (18.5-24.9), overweight (25.0-29.9) and obese (≥30.0). The cumulative oxytocin dose was measured from the start of oxytocin infusion until the neonate was born.

PRIMARY OUTCOME

Cumulative oxytocin dose.

SECONDARY OUTCOME

Maximum rate of oxytocin infusion during the active phase of first stage of labour.

RESULTS

The mean cumulative oxytocin dose increased in the BMI groups (normal weight 2278 mU, overweight 3108 mU and obese 4082 mU (p<0.0001)). However, when adjusted for the confounders (cervical dilatation when oxytocin infusion was started, fetal birth weight, epidural analgesia), the significant difference was no longer seen. The maximum oxytocin infusion rate during the first stage of labour differed significantly in the BMI groups when adjusted for the confounding factors individually but not when adjusted for all three factors simultaneously. In addition, the maximum oxytocin infusion rate was significantly higher in women with emergency caesarean section compared with women with vaginal delivery.

CONCLUSIONS

Women with increasing BMI with augmentation of labour received a higher cumulative oxytocin dose and had a higher maximum oxytocin infusion rate during first stage of labour, however, when adjusted for relevant confounders, the difference was no longer seen. In the future, the guidelines for augmentation of labour with oxytocin infusion might be reconsidered and include modifications related to BMI.