Impact of maternal BMI on intrapartum outcomes: secondary analysis of the Birthplace national prospective cohort study

The aim of this study was to evaluate the impact of maternal BMI on intrapartum interventions and outcomes that may influence the choice of planned place of birth in women with singleton pregnancies of gestation ≥37 weeks planning a vaginal birth. The study focused on outcomes in otherwise healthy women without risk factors other than BMI of 35 of more.

The objectives were:

• to describe the prevalence of medical and obstetric risk factors by maternal BMI
• to evaluate the association between maternal BMI and intrapartum interventions and adverse maternal and perinatal outcomes both overall and in otherwise healthy women without known medical and obstetric risk factors immediately prior to the onset of labour
• to explore whether the effects of BMI differ by parity
• to explore the effect of BMI as a continuous variable on maternal and perinatal outcomes
• to explore the relationships between maternal BMI and maternal and perinatal outcomes in planned home and midwifery unit births

Key findings

• Obesity is associated with increased risks of intrapartum outcomes requiring obstetric or neonatal care, but risks are not the same for all women who are obese.
• Otherwise healthy obese women have an increased risk of augmentation, intrapartum caesarean section and some adverse maternal outcomes, but when interventions and adverse outcomes are considered together, the size of the increased risk is modest (less than 15% for women with a BMI of >35kg/m² compared with women of normal weight).
• Multiparous obese women who do not have additional risk factors are at lower risk of requiring obstetric care during labour and birth than 'low risk' women of normal weight having a first baby.
• Risks to the baby (admission to a neonatal unit or stillbirth/early neonatal death) follow a similar pattern, with lower risks for babies of otherwise healthy multiparous obese women compared with babies of 'low risk' first time mothers of normal weight.

NPEU Contact:

Rachel Rowe (rachel.rowe@npeu.ox.ac.uk)