Rethinking the obesity risk

small_5821992731Research published in BJOG: An International Journal of Obstetrics and Gynaecology shows that women with a high BMI who have previously had at least one baby are less likely to experience obstetric complications during labour and birth than has been thought. The results of this study of 17,230 women without medical or obstetric risk factors showed that, while the risk of interventions requiring obstetric care tended to increase with BMI category, parity was actually much more important as a predictor of absolute risk and the increase in risk for women who are deemed overweight is far more modest than is generally held to be the case.

The study, The impact of maternal obesity on intrapartum outcomes in otherwise low risk women: secondary analysis of the Birthplace national prospective cohort study, is freely available online and is, as you will have gathered from the title, the latest analysis to derive from the Birthplace research.

One element of the findings is particularly worthy of reflection:

[T]he frequency of augmentation of labour suggests that failure to progress may have been the presenting problem in the majority (up to three quarters) of the cases where healthy obese and very obese women experienced outcomes that required obstetric care. This is consistent with a body of evidence indicating that obese women have less effective uterine contractility and longer labours, are more likely to experience failure to progress/labour arrest and have an increased risk of non-elective section for labour arrest disorders or ‘failure to progress / cephalopelvic disproportion’.  (Hollowell et al 2013: 10).

I know that I (amongst many other people) have said it before (see Obesity: naming, blaming, shaming) but there is no direct relationship between higher body weight and ill-health. Obesity may be a risk factor, but the issues are complex and it is impossible to separate out the degree to which it is a risk factor because it truly leads to physiological differences from the degree to which it is a risk factor because people – whether women, clinicians or others – think it is a risk factor.

If you have a moment to spare, please read the quote above again.  The one about women ‘failing to progress’.

To what extent might clinicians’ perceptions that obese women have less effective uterine contractility, longer labours and a greater likelihood of need for obstetric intervention create a self-fulfilling prophecy?

small_7155138495The ‘need’ for augmentation and/or cesarean section is not generally determined by a ‘hard’ measure, such as the woman’s blood pressure reaching a certain threshold. It is generally a subjective decision, and no such decisions are made in a vacuum. They are, instead, made in a context which includes and takes into account relevant factors and issues which may be physical, emotional, sociocultural, epistemological, environmental and/or systemic. They might, for instance, relate to the availability of staff, equipment and facilities. Or attitudes. This context also includes the cultural beliefs of the time, which today include the notion that women with a higher BMI are more likely to have problems. So it is not hard to see how the higher intervention rates experienced by these women may be caused by the fact that we think they are more likely to need intervention.

Which is why it is so helpful to see the authors of this paper recommending a more individualised approach, and suggesting that ‘it may be reasonable to review the BMI criteria for planned birth in non-obstetric unit settings‘ (Hollowell et al 2013: 11).


Sara has also written about similar issues in several of her books.


Hollowell J, Pillas D, Rowe R, Linsell L, Knight M, Brocklehurst P. The impact of maternal obesity on intrapartum outcomes in otherwise low risk women: secondary analysis of the Birthplace national prospective cohort study. BJOG 2013; DOI: 10.1111/1471-0528.12437.

photo credits: rabbit by mattappleby via photopin cc and decision tree by Sasquatch I via photopin cc