Pregnancy, obesity and child asthma: an uncomfortable connection

As a pregnant woman’s weight increases above normal levels, so does her child’s risk of asthma. Flickr/limascarjuliet

Being obese during pregnancy dramatically increases the risk of the child developing asthma by their tenth birthday, according a study published online in the Journal of Allergy and Clinical Immunology.

Researchers from Melbourne and Sweden measured the body mass index (BMI) of almost 130,000 Swedish women in the early stages of pregnancy and found children of very obese women were 57% more likely to develop asthma than children whose mothers were a healthy weight.

The study’s lead author, epidemiologist Dr Adrian Lowe from the Murdoch Childrens Research Institute and the University of Melbourne, explains the impetus for the research and its findings:

The rates of both asthma and obesity have increased dramatically over the past five decades, which raises questions about a possible link between these two epidemics.

One of the consequences of rising rates of obesity in western societies is that mothers are increasingly starting their pregnancies either overweight or obese.

This increases the risks of pre-term birth, the need for interventions such as caesarean sections, as well as the complications of gestational diabetes and hypertension (high blood pressure).

So how does maternal obesity impact on a child’s risk of asthma?

We examined the relationship between the mother’s body mass index (as a marker of overweight or obesity) in the first trimester of pregnancy, along with the risk of her child developing asthma in the first ten years of their life.

We collected information on the height and weight of all new mothers in Stockholm, Sweden between 1998 and 2009, during the first trimester of their pregnancy. We also monitored their children (almost 190,000) to see which ones required asthma medication or hospitalisation for asthma.

We found a very clear, linear pattern: as the mother’s weight increases above the normal range, the risk of asthma in the child increases.

The children of very obese mothers – those with BMI of 35 or above during early pregnancy – had the highest risk of asthma.

But even children of mothers who were slightly overweight – with a BMI of between 25 and 30 – had a small increased risk of asthma when compared with mothers who were in a healthy weight range during early pregnancy.

Even modest amounts of excess weight in early pregnancy increases a child’s risk of ashtma. Flickr/ahhyeah

What are the underlying causes for this link?

There are a number of theories, most of which we can’t directly address within this research.

1) It could be that the child of an obese mother is at increased risk of obesity themselves, and it’s the child’s own obesity that increases their risk of asthma.

2) Obesity is a pro-inflammatory state. So when someone is obese, they have a lot of inflammatory cytokines (immune system regulators) coursing through their system.

Being exposed to this sort of inflammatory milieu during pregnancy might have changed the way the infant’s immune system functions – pushing them toward the development of allergic disease rather than development of tolerance and normal immune function.

3) It could be that maternal obesity during pregnancy alters the metabolic balance of the unborn child, affecting their ability to burn energy and maintain a healthy weight. This relates to the concept of foetal programming, where the inter-uterine environment could affect the foetus’ long-term health.

4) Alternatively, the relationship between the mother’s obesity and the child’s asthma could be due to shared genetic or environmental factors. The lifestyle factors that led mums to become obese – high caloric intake and insufficient exercise – could impact on the child and increase their risk of developing asthma.

What’s next for research in this area?

We’ve just started to study the effects of maternal obesity on childhood asthma by comparing the mother’s weight in two different pregnancies.

Genetics and most environmental exposures are common between two siblings but the mother’s weight can change quite dramatically between two pregnancies.

Studying these differences can help us work out if it really is the mother’s obesity that increases the risk of her child developing asthma or whether it’s something else – a gene or something in the environment.

In determining a child’s risk of astma, we’re asking: Does it matter how heavy the mother is during a particular pregnancy? Or is the mother’s weight acting as a marker for something else?

For our new study, we’re looking at the whole of the Swedish population to see if the increased risk of a child developing asthma is independent of the genetics and common environmental exposures.

The studies are collaborations between Murdoch Childrens Research Institute, the University of Melbourne and Umeå University in Sweden.

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