The impact of maternal obesity on intrapartum outcomes in otherwise low risk women: secondary analysis of the Birthplace national prospective cohort study

Abstract

Objectives: To evaluate the impact of maternal BMI on intrapartum interventions and adverse outcomes that may influence choice of planned birth setting in healthy women without additional risk factors.

Design: Prospective cohort study.

Setting: Stratified random sample of English obstetric units.

Sample: 17,230 women without medical or obstetric risk factors other than obesity.

Methods: Multivariable log Poisson regression was used to evaluate the effect of BMI on risk of intrapartum interventions and adverse maternal and perinatal outcomes adjusted for maternal characteristics.

Main outcome measures: Maternal intervention or adverse outcomes requiring obstetric care (composite of: augmentation, instrumental delivery, intrapartum caesarean section, general anaesthesia, blood transfusion, 3rd/4th degree perineal tear); neonatal unit admission or perinatal death.

Results: In otherwise healthy women, obesity was associated with an increased risk of augmentation, intrapartum caesarean section and some adverse maternal outcomes but when interventions and outcomes requiring obstetric care were considered together, the magnitude of the increased risk was modest (adjusted RR 1.12, 95% CI 1.02-1.23, for BMI > 35 kg/m(2) relative to low risk women of normal weight). Nulliparous low risk women of normal weight had higher absolute risks and were more likely to require obstetric intervention or care than otherwise healthy multiparous women with BMI > 35 kg/m(2) (maternal composite outcome: 53% versus 21%). The perinatal composite outcome exhibited a similar pattern.

Conclusions: Otherwise healthy multiparous obese women may have lower intrapartum risks than previously appreciated. BMI should be considered in conjunction with parity when assessing the potential risks associated with birth in non-obstetric unit settings.

Keywords: Adverse maternal outcomes; adverse perinatal outcomes; augmentation; caesarean section; instrumental delivery; maternal obesity; maternity unit admission criteria; parity; risk factors.

© 2013 The Authors. BJOG published by John Wiley & Sons Ltd on behalf of the Royal College of Obstetricians and Gynaecologists.

Figures

Figure 1

LOWESS plots of perinatal and maternal outcomes by BMI and parity in healthy women without additional risk factors. (A) Admission to a neonatal unit or intrapartum stillbirth/early neonatal death (composite perinatal outcome). (B) Obstetric interventions and adverse maternal outcomes combined (composite maternal outcome). (C) Intrapartum caesarean section. (D) Syntocinon augmentation.For each BMI value on the x-axis, the red line on the LOWESS plot9 shows the unadjusted proportion of births where the outcome occurred. Peaks and troughs caused by small numbers of observations at a particular BMI value are smoothed by using regression methods to take account of values close to the BMI of interest. Individual observations (which take the value one if the event has occurred, zero otherwise) are plotted as black dots at zero and one. The density of the line at zero and one indicates the density of observations at a particular BMI value: the lines become less dense at higher BMI values where observations are sparse.