Virtually every media outlet has reported on the latest study from researchers in the Netherlands, 'Asthma at 8 years of age in children born by cesarean section', in which Roduit et al found an increased prevalence of asthma in children who were born via cesarean delivery. Before I talk about the research itself, I'd just like to point out the biggest misunderstanding of the findings by some reporters. Here is an example:
On December 2nd, Angus Howarth, writing in The Scotsman, began his report with the sentence: "WOMEN who choose to give birth by Caesarean section should be warned it could almost double their baby's chance of developing asthma, researchers said today."
In fact it is ALL pregnant women that should be warned of the risk, including those who choose to give birth vaginally. Why? Because the researchers looked at children born via ALL TYPES of cesarean delivery (e.g. emergency, medical and non-medical), and therefore the risk is actually applicable to all birth methods.
Limitations of the research
The best place to read a more balanced review of this research is here, at the NHS' Behind the headlines website. It reports that: "The study has some limitations, including the broad method it used to diagnose asthma which may not be accurate. In addition, the researchers did not consider several factors known to raise the risk of asthma such as smoking in pregnancy. Further research is needed. Asthma has a range of potential risk factors, both genetic and environmental, but it is unlikely that its sole ‘cause’ is mothers having a caesarean section."
Previous research has found NO LINK between childhood asthma and cesarean delivery
*In 2005, researchers in the U.S., Juhn et al, published 'Mode of delivery at birth and development of asthma: a population-based cohort study.' They followed all children born in Rochester, Minn, between 1976 and 1982, determining the mode of delivery from their birth certificates, and asthma status during the first 7 years of life was ascertained from comprehensive medical record reviews. They found that: "The cumulative incidence rates of asthma among children who were born by cesarean section and vaginal delivery were 3.2% versus 2.6%, 4.6% versus 4.6%, 4.6% versus 5.8%, and 5.7% versus 6.7% at the 1st, 3rd, 5th, and 7th years of life, respectively. The adjusted hazard ratios for cesarean section in predicting asthma and wheezing episode were 0.93 and 0.93 respectively." They concluded: "Mode of delivery is not associated with subsequent risk of developing childhood asthma or wheezing episodes. Because the effect of mode of delivery on a risk of developing asthma or wheezing episodes varies over time (ie, age), selection of the study subjects according to their ages may have influenced the findings of previous studies with a shorter follow-up period."
*In 2004, researchers in the UK, Maitra et al, published 'Mode of delivery is not associated with asthma or atopy in childhood.' The study selected 12,367 children born to mothers resident in a defined area and delivered in one of two major obstetric hospitals between 1991 and 1992, of which 10,980 (88.8%) were delivered vaginally and 1387 (11.2%) by caesarean section. Primary outcomes were parental report of asthma or wheezing between 69 and 81 months of age, physician-diagnosed asthma (PDA) at 91 months of age and atopy at 7 years by skin prick testing. They concluded: "Delivery by caesarean section was not associated with the subsequent development of asthma, wheezing or atopy in later childhood in this population."
But what about the recent Norwegian study that DID find a cesarean/asthma link?
It is true that earlier this year, that in another study, 'Cesarean Section and Risk of Severe Childhood Asthma: A Population-Based Cohort Study', researchers Tollånes et al "found a moderately increased risk of asthma in the children delivered by CS." However, they also write in their conclusion that: "The possibly stronger association with emergency CS compared with planned CS could be worth pursuing to investigate possible causal mechanisms."
Why? Because in their study of 1,756,700 children born between 1967 and 1998 and followed up to age 18 years or the year 2002, those born via emergency cesarean delivery had the greatest risk of asthma. "The prevalence of asthma was 2.3% in the women who delivered by CS, 1.9% in those who had instrumental vaginal delivery, and 1.4% in those who had spontaneous vaginal delivery." This clearly demonstrates (given that the majority of emergency cesareans occur as an outcome of planned vaginal delivery) that healthy women planning cesarean deliveries should not be 'warned' about the risk of asthma any more than other pregnant women.
What else should I know about this latest study from the Netherlands?
The points below are taken from the NHS Knowledge Service's conclusions:
*Asthma is always difficult to diagnose in children. The criteria for diagnosing asthma in this study were broad, and likely to introduce some inaccuracy in the numbers categorised as having asthma. The wheezing and shortness of breath recorded in this study do not necessarily mean that the child has asthma, as they are extremely common in childhood and particularly during viral infections.
*Researchers adjusted their analysis for risk factors related to asthma but several known risk factors were not adjusted for. These include childhood viral infections, smoking during pregnancy and around the child, and number of people living in the house. As highlighted by the results, parental allergy did have considerable effect upon risk estimates.
*The proportions of children born by caesarean section, or who had asthma were relatively small (8.5 and 12.4% respectively), which reduces the power of statistical tests, particularly in the further analysis of subgroups that was conducted.
*Only 70% of children who were included at the beginning of the study completed the eight year follow up and were included in the analysis. The reliability of the results may have been greater had more children completed follow up.
*Women have caesarean sections for many different reasons, including emergencies. The reasons behind having a caesarean delivery may be an important factor in why asthma develops. Possible reasons for any association between childhood asthma and caesarean section have not been clarified by this study and still require further research.
Background info on the Dutch study: 2,917 children participated in a birth cohort study and were followed for 8 years. In total, 12.4% (362) of the children had asthma at the age of 8 years. Cesarean section, with a total prevalence of 8.5%, was associated with an increased risk of asthma, and this association was stronger among predisposed children (with one or more allergic parents) than in children with non-allergic parents. The association between cesarean section and sensitization at the age of 8 years was significant only in children of non-allergic parents.
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