Tuesday December 2 2008
Asthma has a range of risk factors both genetic and environmental
“The rise in asthma… over the past 30 years may have been driven by an increase in caesarean births”, reports The Independent today.
The story is based on Dutch research following almost 3000 children from birth to the age of eight, which looked at methods of birth and the risk of having asthma. It explored a theory that during a caesarean babies are not exposed to bacteria that would normally kick-start their immune system, and that this can lead to allergic conditions such as asthma.
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.
Where did the story come from?
This research was carried out by C Roduit from the National Institute for Risk Assessment Sciences, and colleagues from other institutions in the Netherlands.
The study was funded by the Netherlands Organisation for Health Research and Development, the Netherlands Asthma Foundation and the Netherlands Ministry of Health, Welfare and Support. It was published in the peer-reviewed medical journal, Thorax.
What kind of scientific study was this?
This was a cohort study investigating whether children delivered by caesarean section were more at risk of developing childhood asthma. Over the past decade, the incidence of both asthma and caesarean deliveries has risen in the Netherlands, where the study was conducted.
There is a theory that babies born by caesarean may have delayed exposure to microbes compared to those born vaginally, affecting their immune system and, in turn, leading to allergic responses such as asthma.
In the Prevention and Incidence of Asthma and Mite Allergy (PIAMA) study, 4146 pregnant women had their children followed up to the age of eight years. Thirty-two percent of these women were categorised as being allergic and the rest were non-allergic based on their responses to common allergens.
From all the children in the study, 1327 with an allergic mother and 663 with a non-allergic mother were selected for closer follow-up. This included blood tests measuring their allergic response to certain common allergens. Questionnaires were given to the mothers prior to delivery, three months following delivery and then annually from one year of age.
At the age of eight years, full data was available on 2917 of the total 4146 children in the study. Of the children selected for close follow-up, 1703 had full blood test data.
Asthma was diagnosed if in any of the annual questionnaires, parents reported one or more attacks of wheeze, shortness of breath or prescription of inhaled steroids in the past 12 months. Chronic asthma was defined if there was ‘asthma’ at any time between three and seven years and also at eight years.
Information was also obtained on other potential risk factors for asthma, including birth details and whether the child was breastfed, number of siblings in the family, smoking during pregnancy, maternal education, maternal age and maternal height and weight.
The researchers assessed the association between asthma at age eight and method of delivery while adjusting for the most relevant of the other risk factors.
What were the results of the study?
Of the total children followed-up, 51% had non-allergic parents, 40% had one allergic parent and 9% had two allergic parents. Of the total 2917 children with full data, 8.5% were born by caesarean section. Among children born by caesarean section, low birth weight, growth restriction and prematurity were more common than in those born vaginally.
Of the total children in the study, 12.4% (362 children) had asthma at eight years old. Asthma was higher among children with one or more allergic parents than with two non-allergic parents. Most children with an asthma diagnosis aged eight, also had a diagnosis between three and seven years. Caesarean section increased the risk of asthma (odds ratio 1.79, 95% confidence interval 1.27 to 2.51).
In sub-group analysis, this association was even stronger in those children with one allergic parent (OR 1.86) or two allergic parents (OR 2.91) and was actually not statistically significant in children with non-allergic parents (OR 1.36). Blood tests at age eight found there was only a significant association between caesarean section and allergic sensitisation in children with non-allergic parents (OR 2.14).
What interpretations did the researchers draw from these results?
The researchers made the conclusion that children born by caesarean section have a higher risk of developing asthma, particularly if they have allergic parents.
What does the NHSKnowledge Service make of this study?
This study has suggested a possible link between childhood asthma and caesarean section. Although the prospective study design does add strength to findings on the subject, there are several important limitations to consider:
- 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.
Sir Muir Gray adds...
More research is needed.