Maternal obesity is a significant risk factor for many complications during pregnancy and birth including gestational diabetes, miscarriage, stillbirth, caesarean section and instrumental delivery.
Current health guidelines recommend that women with a body mass index (BMI) of more than 35 should be advised to give birth in an obstetric unit to reduce the risks of adverse maternal and fetal outcomes. These guidelines currently apply to all obese women, even in the absence of other medical or obstetric risk factors associated with obesity (such as diabetes and hypertension).
This new study, of 17,230 women without medical or obstetric risk factors other than obesity, focuses on the impact of BMI on birth interventions and complications resulting in the need for an obstetric unit birth.
Women in the study, from the Birthplace in England national cohort, were categorised in groups according to BMI, including underweight (BMI <18.5), normal weight (BMI 18.5-24.9), overweight (BMI 25-29.9), obese (BMI 30-35) and very obese (BMI >35).
The study looked to see how often women experienced events during birth that would require obstetric or neonatal care. These included augmentation of labour, instrumental delivery, intrapartum caesarean section, blood transfusion and maternal admission for higher level care, while for the baby the study looked at admission to a neonatal unit, intrapartum stillbirth (stillbirth during labour) or early neonatal death.
Researchers found that while the risk of interventions requiring obstetric care tended to increase with BMI category, parity (the number of times a women has previously given birth) was a much more important predictor of absolute risk.
The findings showed that healthy nulliparous (first-time) mothers of normal weight were more likely to require maternal obstetric intervention than healthy multiparous (at least one previous birth) mothers who were classed as very obese, 53% vs 21%.
The incidence of adverse fetal outcomes, including admissions to a neonatal unit, intrapartum stillbirth and early neonatal death, was found to increase with BMI category, from 2.4% in underweight women to 4.7% in very obese women. Again, in each BMI category the risks were much lower in otherwise low-risk women who had previously had a baby compared with those giving birth for the first time.
Jennifer Hollowell, National Perinatal Epidemiology Unit, University of Oxford and co-author of the paper, said:
“Obesity is associated with increased risks to the mother and baby during labour and birth, but we found that these risks were not the same for all obese women and that the increased risk was fairly modest for obese women who did not have additional medical or obstetric risk factors.
“Interestingly, our results showed that obese women who have previously had a baby and who do not have additional risk factors such as diabetes or a previous caesarean section, may have lower obstetric risks than previously appreciated. In particular, the absolute risks that the mother or baby will require obstetric or neonatal care was lower in this group than for low-risk women of normal weight having a first baby.
“These findings suggest that it may be reasonable to allow some multiparous obese women, who are otherwise healthy, a choice of birth setting.”
Mike Marsh, BJOG editor-in-chief, added:
“We know that maternal obesity is a big problem facing clinicians in this country as it is a major risk factor for many obstetric complications including gestational diabetes, miscarriage, caesarean section and stillbirth.
“This large-scale study confirms the concerns around obesity and the increased likelihood of adverse outcomes for the baby. However, it also suggests that parity may have a modifying effect on the risks associated with obesity in otherwise low-risk pregnant women.
“While obstetricians need to consider parity as a modifying factor when assessing the potential risks associated with pregnancy outcomes, deviations from guidelines concerning birth setting in obese mothers should be made on a case by case basis.”
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BJOG: An International Journal of Obstetrics and Gynaecology is owned by the Royal College of Obstetricians and Gynaecologists (RCOG) but is editorially independent and published monthly by Wiley-Blackwell. The journal features original, peer-reviewed, high-quality medical research in all areas of obstetrics and gynaecology worldwide. Please quote ‘BJOG’ or ‘BJOG: An International Journal of Obstetrics and Gynaecology’ when referring to the journal and include the website: www.bjog.org as a hidden link online.
Please include a link to the paper in online coverage: http://dx.doi.org/10.1111/1471-0528.12437
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Jennifer Hollowell, Demetris Pillas, Rachel Rowe, Louise Linsell, Marian Knight, Peter Brocklehurst. The impact of maternal obesity on intrapartum outcomes in otherwise low risk women: secondary analysis of the Birthplace national prospective cohort study. BJOG 2013; 10.1111/1471-0528.12437