C-sections unlikely to cause asthma


3 Dec 2015

A large observational study published in JAMA this week found a small increase in asthma risk with planned C-section delivery compared to vaginal delivery. Here’s what the experts had to say about the findings. 

Professor Jonathan Morris is Professor of Obstetrics and Gynaecology at the University of Sydney and Director of the Kolling Institute of Medical Research

 “This paper confirms other reports that caesarean is associated with asthma. The association with planned caesarean does not agree with others that have explored this question. The findings are associative only and certainly should not be interpreted that caesarean causes asthma. However, for many reasons it is always wise to consider the risks and benefits of caesarean over planned normal birth.”

Commenting on the asthma aspect) Prof Seif Shaheen, Clinical Professor of Respiratory Epidemiology, Queen Mary University of London (QMUL), said:

 “Many epidemiological studies have investigated a link between caesarean section and childhood asthma, but the evidence has been conflicting. This latest data-linkage study from Scotland, which is very large, reports an increased risk of asthma associated with planned C-section delivery, compared with vaginal delivery. I am puzzled why they did not also report results comparing unscheduled C-section delivery versus vaginal delivery. There are two key questions to consider in interpreting these findings:

“1. Is this association likely to be causal?  In my view there are a number of reasons to doubt this interpretation: First, a key potential confounder*, namely maternal body mass index (BMI), was not controlled for in the analyses – women having a C-section had a higher BMI than women delivering vaginally (see Table 1), and a higher maternal BMI in pregnancy is associated with an increased risk of asthma in the offspring. Second, when the analysis was restricted to women with no missing data on potential confounders, planned C-section was no longer associated with an increased risk of hospital admission for asthma. Third, two Swedish studies, which used a more sophisticated methodology to rule out potential confounding, found no evidence of association between planned C-section and asthma in children over five years of age.

“2. Even if the link between planned C-section and asthma is causal, is it important and should pregnant women be concerned?  As the magnitude of increased risk was small, and the proportion of women in this study having a planned C-section was also small (less than 4%), this means that the proportion of asthma in the population that might be attributable to planned C-section (assuming a causal link) will also be small.

 “In summary, evidence for a causal association is not strong, these findings should be treated with caution, and pregnant women should be reassured.”

Prof Jean Golding, Emeritus Professor of Paediatric and Perinatal Epidemiology, University of Bristol, said:

 “It is very important to try to assess whether Caesarean section has any long term effects on the developing child. In this paper, however, it is important to note that the authors were unable to determine the reasons why the mothers in this study were having a C-section. The very slight increases in risk found may be related to these reasons rather than the section itself. Unless the reasons can be identified and taken into account I feel that the findings in this paper should not be used when making a decision as to whether a C-section should be undertaken.”

 Prof Kevin McConway, Professor of Applied Statistics, The Open University, said:

 “In statistical terms, the researchers have gone a long way in trying to sort out what is related to what. But the huge size of the study – more than 300,000 births over a period fifteen years – is itself an indication of how difficult it is to make sense of this subject. What makes it difficult is that, happily, the great majority of children do not suffer from the diseases and adverse events that were studied. Many thousands of babies have to be studied before the researchers can observe enough cases of asthma or diabetes to draw conclusions at all. Furthermore, the risks of all these diseases are affected by many other things apart from whether the children were delivered by caesarean section or vaginally. While these researchers used appropriate statistical methods to allow for other factors as far as possible, such corrections cannot be perfect, and in any case they can’t allow for things that they have no data on.

“Dr Black, the lead researcher, can appropriately say that “we are confident in saying the delivery could be playing a role in the development of asthma”. But please note the “could”. Dr Black, rightly, emphasises that they cannot say that planned C-section definitely causes a small increase in the risk of asthma. Maybe it does, or maybe the increase is due to something other than the delivery that could not be allowed for statistically. The research report points out that they could not allow statistically for reasons why a C-section was chosen. Maybe it’s one of the reasons for choosing a C-section that leads to the increased risk of asthma, and not the C-section at all. The same goes for the observation that there was a very small increase in the death rate up to age 21 of those born by planned C-section compared to those born vaginally. That could have something to do with the method of delivery, but there are many other possible explanations. 

“Further, these results are all for first births, and it remains possible that things are different for subsequent births.

“We mustn’t forget these uncertainties, particularly as the possible increase in risk is really rather small. This research found that, for 1000 children born vaginally, 34 developed asthma requiring hospital admission.  For 1000 born by planned caesarean, 37 developed such asthma. Not a huge difference, even if the method of delivery is the cause. And as the researchers point out clearly, if there’s a medical reason why caesarean delivery is preferred, this small increased risk isn’t going to be enough to overturn that preference.

“For me, a key point in the research paper is its last sentence, “Further investigation is needed to understand whether the observed associations are causal”. But that further investigation is likely also to involve thousands of births and many years of further work.”

*Comments gathered by UK SMC*

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